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# 2.4   Space Complexity

The space complexity is used to measure the growth trend of memory consumption as the scale of data increases for an algorithm solution. This concept is analogous to time complexity by replacing "runtime" with "memory space".

## 2.4.1   Algorithmic Correlation Space

The memory space used by algorithms during its execution include the following types.

- **Input Space**: Used to store the input data for the algorithm.
- **Temporary Space**: Used to store variables, objects, function contexts, and other data of the algorithm during runtime.
- **Output Space**: Used to store the output data of the algorithm.

In general, the "Input Space" is excluded from the statistics of space complexity.

The **Temporary Space** can be further divided into three parts.

- **Temporary Data**: Used to store various constants, variables, objects, etc., during the the algorithm's execution.
- **Stack Frame Space**: Used to hold the context data of the called function. The system creates a stack frame at the top of the stack each time a function is called, and the stack frame space is freed when the function returns.
- **Instruction Space**: Used to hold compiled program instructions, usually ignored in practical statistics.

When analyzing the space complexity of a piece of program, **three parts are usually taken into account: Temporary Data, Stack Frame Space and Output Data**.

![Associated spaces used by the algorithm](space_complexity.assets/space_types.png){ class="animation-figure" }

<p align="center"> Figure 2-15 &nbsp; Associated spaces used by the algorithm </p>

=== "Python"

    ```python title=""
    class Node:
        """Classes""""
        def __init__(self, x: int):
            self.val: int = x               # node value
            self.next: Node | None = None   # reference to the next node

    def function() -> int:
        """"Functions"""""
        # Perform certain operations...
        return 0

    def algorithm(n) -> int:    # input data
        A = 0                   # temporary data (constant, usually in uppercase)
        b = 0                   # temporary data (variable)
        node = Node(0)          # temporary data (object)
        c = function()          # Stack frame space (call function)
        return A + b + c        # output data
    ```

=== "C++"

    ```cpp title=""
    /* Structures */
    struct Node {
        int val;
        Node *next;
        Node(int x) : val(x), next(nullptr) {}
    };

    /* Functions */
    int func() {
        // Perform certain operations...
        return 0;
    }

    int algorithm(int n) {          // input data
        const int a = 0;            // temporary data (constant)
        int b = 0;                  // temporary data (variable)
        Node* node = new Node(0);   // temporary data (object)
        int c = func();             // stack frame space (call function)
        return a + b + c;           // output data
    }
    ```

=== "Java"

    ```java title=""
    /* Classes */
    class Node {
        int val;
        Node next;
        Node(int x) { val = x; }
    }
   
    /* Functions */
    int function() {
        // Perform certain operations...
        return 0;
    }
   
    int algorithm(int n) {          // input data
        final int a = 0;            // temporary data (constant)
        int b = 0;                  // temporary data (variable)
        Node node = new Node(0);    // temporary data (object)
        int c = function();         // stack frame space (call function)
        return a + b + c;           // output data
    }
    ```

=== "C#"

    ```csharp title=""
    /* Classes */
    class Node {
        int val;
        Node next;
        Node(int x) { val = x; }
    }

    /* Functions */
    int Function() {
        // Perform certain operations...
        return 0;
    }

    int Algorithm(int n) {  // input data
        const int a = 0;    // temporary data (constant)
        int b = 0;          // temporary data (variable)
        Node node = new(0); // temporary data (object)
        int c = Function(); // stack frame space (call function)
        return a + b + c;   // output data
    }
    ```

=== "Go"

    ```go title=""
    /* Structures */
    type node struct {
        val  int
        next *node
    }

    /* Create node structure */
    func newNode(val int) *node {
        return &node{val: val}
    }
   
    /* Functions */
    func function() int {
        // Perform certain operations...
        return 0
    }

    func algorithm(n int) int { // input data
        const a = 0             // temporary data (constant)
        b := 0                  // temporary storage of data (variable)
        newNode(0)              // temporary data (object)
        c := function()         // stack frame space (call function)
        return a + b + c        // output data
    }
    ```

=== "Swift"

    ```swift title=""
    /* Classes */
    class Node {
        var val: Int
        var next: Node?

        init(x: Int) {
            val = x
        }
    }

    /* Functions */
    func function() -> Int {
        // Perform certain operations...
        return 0
    }

    func algorithm(n: Int) -> Int { // input data
        let a = 0                   // temporary data (constant)
        var b = 0                   // temporary data (variable)
        let node = Node(x: 0)       // temporary data (object)
        let c = function()          // stack frame space (call function)
        return a + b + c            // output data
    }
    ```

=== "JS"

    ```javascript title=""
    /* Classes */
    class Node {
        val;
        next;
        constructor(val) {
            this.val = val === undefined ? 0 : val; // node value
            this.next = null;                       // reference to the next node
        }
    }

    /* Functions */
    function constFunc() {
        // Perform certain operations
        return 0;
    }

    function algorithm(n) {         // input data
        const a = 0;                // temporary data (constant)
        let b = 0;                  // temporary data (variable)
        const node = new Node(0);   // temporary data (object)
        const c = constFunc();      // Stack frame space (calling function)
        return a + b + c;           // output data
    }
    ```

=== "TS"

    ```typescript title=""
    /* Classes */
    class Node {
        val: number;
        next: Node | null;
        constructor(val?: number) {
            this.val = val === undefined ? 0 : val; // node value
            this.next = null;                       // reference to the next node
        }
    }

    /* Functions */
    function constFunc(): number {
        // Perform certain operations
        return 0;
    }

    function algorithm(n: number): number { // input data
        const a = 0;                        // temporary data (constant)
        let b = 0;                          // temporary data (variable)
        const node = new Node(0);           // temporary data (object)
        const c = constFunc();              // Stack frame space (calling function)
        return a + b + c;                   // output data
    }
    ```

=== "Dart"

    ```dart title=""
    /* Classes */
    class Node {
      int val;
      Node next;
      Node(this.val, [this.next]);
    }

    /* Functions */
    int function() {
      // Perform certain operations...
      return 0;
    }

    int algorithm(int n) {  // input data
      const int a = 0;      // temporary data (constant)
      int b = 0;            // temporary data (variable)
      Node node = Node(0);  // temporary data (object)
      int c = function();   // stack frame space (call function)
      return a + b + c;     // output data
    }
    ```

=== "Rust"

    ```rust title=""
    use std::rc::Rc;
    use std::cell::RefCell;
   
    /* Structures */
    struct Node {
        val: i32,
        next: Option<Rc<RefCell<Node>>>,
    }

    /* Creating a Node structure */
    impl Node {
        fn new(val: i32) -> Self {
            Self { val: val, next: None }
        }
    }

    /* Functions */
    fn function() -> i32 {     
        // Perform certain operations...
        return 0;
    }

    fn algorithm(n: i32) -> i32 {   // input data
        const a: i32 = 0;           // temporary data (constant)
        let mut b = 0;              // temporary data (variable)
        let node = Node::new(0);    // temporary data (object)
        let c = function();         // stack frame space (call function)
        return a + b + c;           // output data
    }
    ```

=== "C"

    ```c title=""
    /* Functions */
    int func() {
        // Perform certain operations...
        return 0;
    }

    int algorithm(int n) {  // input data
        const int a = 0;    // temporary data (constant)
        int b = 0;          // temporary data (variable)
        int c = func();     // stack frame space (call function)
        return a + b + c;   // output data
    }
    ```

=== "Zig"

    ```zig title=""

    ```

## 2.4.2 &nbsp; Calculation Method

The calculation method for space complexity is pretty similar to time complexity, with the only difference being that the focus shifts from "operation count" to "space usage size".

On top of that, unlike time complexity, **we usually only focus on the worst-case space complexity**. This is because memory space is a hard requirement, and we have to make sure that there is enough memory space reserved for all possibilities incurred by input data.

Looking at the following code, the "worst" in worst-case space complexity has two layers of meaning.

1. **Based on the worst-case input data**: when $n < 10$, the space complexity is $O(1)$; however, when $n > 10$, the initialized array `nums` occupies $O(n)$ space; thus the worst-case space complexity is $O(n)$.
2. **Based on the peak memory during algorithm execution**: for example, the program occupies $O(1)$ space until the last line is executed; when the array `nums` is initialized, the program occupies $O(n)$ space; thus the worst-case space complexity is $O(n)$.

=== "Python"

    ```python title=""
    def algorithm(n: int):
        a = 0               # O(1)
        b = [0] * 10000     # O(1)
        if n > 10:
            nums = [0] * n  # O(n)
    ```

=== "C++"

    ```cpp title=""
    void algorithm(int n) {
        int a = 0;               // O(1)
        vector<int> b(10000);    // O(1)
        if (n > 10)
            vector<int> nums(n); // O(n)
    }
    ```

=== "Java"

    ```java title=""
    void algorithm(int n) {
        int a = 0;                   // O(1)
        int[] b = new int[10000];    // O(1)
        if (n > 10)
            int[] nums = new int[n]; // O(n)
    }
    ```

=== "C#"

    ```csharp title=""
    void Algorithm(int n) {
        int a = 0;                   // O(1)
        int[] b = new int[10000];    // O(1)
        if (n > 10) {
            int[] nums = new int[n]; // O(n)
        }
    }
    ```

=== "Go"

    ```go title=""
    func algorithm(n int) {
        a := 0                      // O(1)
        b := make([]int, 10000)     // O(1)
        var nums []int
        if n > 10 {
            nums := make([]int, n)  // O(n)
        }
        fmt.Println(a, b, nums)
    }
    ```

=== "Swift"

    ```swift title=""
    func algorithm(n: Int) {
        let a = 0 // O(1)
        let b = Array(repeating: 0, count: 10000) // O(1)
        if n > 10 {
            let nums = Array(repeating: 0, count: n) // O(n)
        }
    }
    ```

=== "JS"

    ```javascript title=""
    function algorithm(n) {
        const a = 0;                   // O(1)
        const b = new Array(10000);    // O(1)
        if (n > 10) {
            const nums = new Array(n); // O(n)
        }
    }
    ```

=== "TS"

    ```typescript title=""
    function algorithm(n: number): void {
        const a = 0;                   // O(1)
        const b = new Array(10000);    // O(1)
        if (n > 10) {
            const nums = new Array(n); // O(n)
        }
    }
    ```

=== "Dart"

    ```dart title=""
    void algorithm(int n) {
      int a = 0;                            // O(1)
      List<int> b = List.filled(10000, 0);  // O(1)
      if (n > 10) {
        List<int> nums = List.filled(n, 0); // O(n)
      }
    }
    ```

=== "Rust"

    ```rust title=""
    fn algorithm(n: i32) {
        let a = 0;                              // O(1)
        let b = [0; 10000];                     // O(1)
        if n > 10 {
            let nums = vec![0; n as usize];     // O(n)
        }
    }
    ```

=== "C"

    ```c title=""
    void algorithm(int n) {
        int a = 0;               // O(1)
        int b[10000];            // O(1)
        if (n > 10)
            int nums[n] = {0};   // O(n)
    }
    ```

=== "Zig"

    ```zig title=""

    ```

**In recursion functions, it is important to take into count the measurement of stack frame space**. For example in the following code:

- The function `loop()` calls $n$ times `function()` in a loop, and each round of `function()` returns and frees stack frame space, so the space complexity is still $O(1)$.
- The recursion function `recur()` will have $n$ unreturned `recur()` during runtime, thus occupying $O(n)$ of stack frame space.

=== "Python"

    ```python title=""
    def function() -> int:
        # Perform certain operations
        return 0

    def loop(n: int):
        """Loop O(1)"""""
        for _ in range(n):
            function()

    def recur(n: int) -> int:
        """Recursion O(n)"""""
        if n == 1: return
        return recur(n - 1)
    ```

=== "C++"

    ```cpp title=""
    int func() {
        // Perform certain operations
        return 0;
    }
    /* Cycle O(1) */
    void loop(int n) {
        for (int i = 0; i < n; i++) {
            func();
        }
    }
    /* Recursion O(n) */
    void recur(int n) {
        if (n == 1) return;
        return recur(n - 1);
    }
    ```

=== "Java"

    ```java title=""
    int function() {
        // Perform certain operations
        return 0;
    }
    /* Cycle O(1) */
    void loop(int n) {
        for (int i = 0; i < n; i++) {
            function();
        }
    }
    /* Recursion O(n) */
    void recur(int n) {
        if (n == 1) return;
        return recur(n - 1);
    }
    ```

=== "C#"

    ```csharp title=""
    int Function() {
        // Perform certain operations
        return 0;
    }
    /* Cycle O(1) */
    void Loop(int n) {
        for (int i = 0; i < n; i++) {
            Function();
        }
    }
    /* Recursion O(n) */
    int Recur(int n) {
        if (n == 1) return 1;
        return Recur(n - 1);
    }
    ```

=== "Go"

    ```go title=""
    func function() int {
        // Perform certain operations
        return 0
    }
   
    /* Cycle O(1) */
    func loop(n int) {
        for i := 0; i < n; i++ {
            function()
        }
    }
   
    /* Recursion O(n) */
    func recur(n int) {
        if n == 1 {
            return
        }
        recur(n - 1)
    }
    ```

=== "Swift"

    ```swift title=""
    @discardableResult
    func function() -> Int {
        // Perform certain operations
        return 0
    }

    /* Cycle O(1) */
    func loop(n: Int) {
        for _ in 0 ..< n {
            function()
        }
    }

    /* Recursion O(n) */
    func recur(n: Int) {
        if n == 1 {
            return
        }
        recur(n: n - 1)
    }
    ```

=== "JS"

    ```javascript title=""
    function constFunc() {
        // Perform certain operations
        return 0;
    }
    /* Cycle O(1) */
    function loop(n) {
        for (let i = 0; i < n; i++) {
            constFunc();
        }
    }
    /* Recursion O(n) */
    function recur(n) {
        if (n === 1) return;
        return recur(n - 1);
    }
    ```

=== "TS"

    ```typescript title=""
    function constFunc(): number {
        // Perform certain operations
        return 0;
    }
    /* Cycle O(1) */
    function loop(n: number): void {
        for (let i = 0; i < n; i++) {
            constFunc();
        }
    }
    /* Recursion O(n) */
    function recur(n: number): void {
        if (n === 1) return;
        return recur(n - 1);
    }
    ```

=== "Dart"

    ```dart title=""
    int function() {
      // Perform certain operations
      return 0;
    }
    /* Cycle O(1) */
    void loop(int n) {
      for (int i = 0; i < n; i++) {
        function();
      }
    }
    /* Recursion O(n) */
    void recur(int n) {
      if (n == 1) return;
      return recur(n - 1);
    }
    ```

=== "Rust"

    ```rust title=""
    fn function() -> i32 {
        // Perform certain operations
        return 0;
    }
    /* Cycle O(1) */
    fn loop(n: i32) {
        for i in 0..n {
            function();
        }
    }
    /* Recursion O(n) */
    void recur(n: i32) {
        if n == 1 {
            return;
        }
        recur(n - 1);
    }
    ```

=== "C"

    ```c title=""
    int func() {
        // Perform certain operations
        return 0;
    }
    /* Cycle O(1) */
    void loop(int n) {
        for (int i = 0; i < n; i++) {
            func();
        }
    }
    /* Recursion O(n) */
    void recur(int n) {
        if (n == 1) return;
        return recur(n - 1);
    }
    ```

=== "Zig"

    ```zig title=""

    ```

## 2.4.3 &nbsp; Common Types

Assuming the input data size is $n$, the figure illustrates common types of space complexity (ordered from low to high).

$$
\begin{aligned}
O(1) < O(\log n) < O(n) < O(n^2) < O(2^n) \newline
\text{constant order} < \text{logarithmic order} < \text{linear order} < \text{square order} < \text{exponential order}
\end{aligned}
$$

![Common space complexity types](space_complexity.assets/space_complexity_common_types.png){ class="animation-figure" }

<p align="center"> Figure 2-16 &nbsp; Common space complexity types </p>

### 1. &nbsp; Constant Order $O(1)$

Constant order is common for constants, variables, and objects whose quantity is unrelated to the size of the input data $n$.

It is important to note that memory occupied by initializing a variable or calling a function in a loop is released once the next iteration begins. Therefore, there is no accumulation of occupied space and the space complexity remains $O(1)$ :

=== "Python"

    ```python title="space_complexity.py"
    def function() -> int:
        """函数"""
        # 执行某些操作
        return 0

    def constant(n: int):
        """常数阶"""
        # 常量、变量、对象占用 O(1) 空间
        a = 0
        nums = [0] * 10000
        node = ListNode(0)
        # 循环中的变量占用 O(1) 空间
        for _ in range(n):
            c = 0
        # 循环中的函数占用 O(1) 空间
        for _ in range(n):
            function()
    ```

=== "C++"

    ```cpp title="space_complexity.cpp"
    /* 函数 */
    int func() {
        // 执行某些操作
        return 0;
    }

    /* 常数阶 */
    void constant(int n) {
        // 常量、变量、对象占用 O(1) 空间
        const int a = 0;
        int b = 0;
        vector<int> nums(10000);
        ListNode node(0);
        // 循环中的变量占用 O(1) 空间
        for (int i = 0; i < n; i++) {
            int c = 0;
        }
        // 循环中的函数占用 O(1) 空间
        for (int i = 0; i < n; i++) {
            func();
        }
    }
    ```

=== "Java"

    ```java title="space_complexity.java"
    /* 函数 */
    int function() {
        // 执行某些操作
        return 0;
    }

    /* 常数阶 */
    void constant(int n) {
        // 常量、变量、对象占用 O(1) 空间
        final int a = 0;
        int b = 0;
        int[] nums = new int[10000];
        ListNode node = new ListNode(0);
        // 循环中的变量占用 O(1) 空间
        for (int i = 0; i < n; i++) {
            int c = 0;
        }
        // 循环中的函数占用 O(1) 空间
        for (int i = 0; i < n; i++) {
            function();
        }
    }
    ```

=== "C#"

    ```csharp title="space_complexity.cs"
    /* 函数 */
    int Function() {
        // 执行某些操作
        return 0;
    }

    /* 常数阶 */
    void Constant(int n) {
        // 常量、变量、对象占用 O(1) 空间
        int a = 0;
        int b = 0;
        int[] nums = new int[10000];
        ListNode node = new(0);
        // 循环中的变量占用 O(1) 空间
        for (int i = 0; i < n; i++) {
            int c = 0;
        }
        // 循环中的函数占用 O(1) 空间
        for (int i = 0; i < n; i++) {
            Function();
        }
    }
    ```

=== "Go"

    ```go title="space_complexity.go"
    /* 函数 */
    func function() int {
        // 执行某些操作...
        return 0
    }

    /* 常数阶 */
    func spaceConstant(n int) {
        // 常量、变量、对象占用 O(1) 空间
        const a = 0
        b := 0
        nums := make([]int, 10000)
        ListNode := newNode(0)
        // 循环中的变量占用 O(1) 空间
        var c int
        for i := 0; i < n; i++ {
            c = 0
        }
        // 循环中的函数占用 O(1) 空间
        for i := 0; i < n; i++ {
            function()
        }
        fmt.Println(a, b, nums, c, ListNode)
    }
    ```

=== "Swift"

    ```swift title="space_complexity.swift"
    /* 函数 */
    @discardableResult
    func function() -> Int {
        // 执行某些操作
        return 0
    }

    /* 常数阶 */
    func constant(n: Int) {
        // 常量、变量、对象占用 O(1) 空间
        let a = 0
        var b = 0
        let nums = Array(repeating: 0, count: 10000)
        let node = ListNode(x: 0)
        // 循环中的变量占用 O(1) 空间
        for _ in 0 ..< n {
            let c = 0
        }
        // 循环中的函数占用 O(1) 空间
        for _ in 0 ..< n {
            function()
        }
    }
    ```

=== "JS"

    ```javascript title="space_complexity.js"
    /* 函数 */
    function constFunc() {
        // 执行某些操作
        return 0;
    }

    /* 常数阶 */
    function constant(n) {
        // 常量、变量、对象占用 O(1) 空间
        const a = 0;
        const b = 0;
        const nums = new Array(10000);
        const node = new ListNode(0);
        // 循环中的变量占用 O(1) 空间
        for (let i = 0; i < n; i++) {
            const c = 0;
        }
        // 循环中的函数占用 O(1) 空间
        for (let i = 0; i < n; i++) {
            constFunc();
        }
    }
    ```

=== "TS"

    ```typescript title="space_complexity.ts"
    /* 函数 */
    function constFunc(): number {
        // 执行某些操作
        return 0;
    }

    /* 常数阶 */
    function constant(n: number): void {
        // 常量、变量、对象占用 O(1) 空间
        const a = 0;
        const b = 0;
        const nums = new Array(10000);
        const node = new ListNode(0);
        // 循环中的变量占用 O(1) 空间
        for (let i = 0; i < n; i++) {
            const c = 0;
        }
        // 循环中的函数占用 O(1) 空间
        for (let i = 0; i < n; i++) {
            constFunc();
        }
    }
    ```

=== "Dart"

    ```dart title="space_complexity.dart"
    /* 函数 */
    int function() {
      // 执行某些操作
      return 0;
    }

    /* 常数阶 */
    void constant(int n) {
      // 常量、变量、对象占用 O(1) 空间
      final int a = 0;
      int b = 0;
      List<int> nums = List.filled(10000, 0);
      ListNode node = ListNode(0);
      // 循环中的变量占用 O(1) 空间
      for (var i = 0; i < n; i++) {
        int c = 0;
      }
      // 循环中的函数占用 O(1) 空间
      for (var i = 0; i < n; i++) {
        function();
      }
    }
    ```

=== "Rust"

    ```rust title="space_complexity.rs"
    /* 函数 */
    fn function() ->i32 {
        // 执行某些操作
        return 0;
    }

    /* 常数阶 */
    #[allow(unused)]
    fn constant(n: i32) {
        // 常量、变量、对象占用 O(1) 空间
        const A: i32 = 0;
        let b = 0;
        let nums = vec![0; 10000];
        let node = ListNode::new(0);
        // 循环中的变量占用 O(1) 空间
        for i in 0..n {
            let c = 0;
        }
        // 循环中的函数占用 O(1) 空间
        for i in 0..n {
            function();
        }
    }
    ```

=== "C"

    ```c title="space_complexity.c"
    /* 函数 */
    int func() {
        // 执行某些操作
        return 0;
    }

    /* 常数阶 */
    void constant(int n) {
        // 常量、变量、对象占用 O(1) 空间
        const int a = 0;
        int b = 0;
        int nums[1000];
        ListNode *node = newListNode(0);
        free(node);
        // 循环中的变量占用 O(1) 空间
        for (int i = 0; i < n; i++) {
            int c = 0;
        }
        // 循环中的函数占用 O(1) 空间
        for (int i = 0; i < n; i++) {
            func();
        }
    }
    ```

=== "Zig"

    ```zig title="space_complexity.zig"
    // 函数
    fn function() i32 {
        // 执行某些操作
        return 0;
    }

    // 常数阶
    fn constant(n: i32) void {
        // 常量、变量、对象占用 O(1) 空间
        const a: i32 = 0;
        var b: i32 = 0;
        var nums = [_]i32{0}**10000;
        var node = inc.ListNode(i32){.val = 0};
        var i: i32 = 0;
        // 循环中的变量占用 O(1) 空间
        while (i < n) : (i += 1) {
            var c: i32 = 0;
            _ = c;
        }
        // 循环中的函数占用 O(1) 空间
        i = 0;
        while (i < n) : (i += 1) {
            _ = function();
        }
        _ = a;
        _ = b;
        _ = nums;
        _ = node;
    }
    ```

### 2. &nbsp; Linear Order $O(N)$

Linear order is commonly found in arrays, linked lists, stacks, queues, and similar structures where the number of elements is proportional to $n$:

=== "Python"

    ```python title="space_complexity.py"
    def linear(n: int):
        """线性阶"""
        # 长度为 n 的列表占用 O(n) 空间
        nums = [0] * n
        # 长度为 n 的哈希表占用 O(n) 空间
        hmap = dict[int, str]()
        for i in range(n):
            hmap[i] = str(i)
    ```

=== "C++"

    ```cpp title="space_complexity.cpp"
    /* 线性阶 */
    void linear(int n) {
        // 长度为 n 的数组占用 O(n) 空间
        vector<int> nums(n);
        // 长度为 n 的列表占用 O(n) 空间
        vector<ListNode> nodes;
        for (int i = 0; i < n; i++) {
            nodes.push_back(ListNode(i));
        }
        // 长度为 n 的哈希表占用 O(n) 空间
        unordered_map<int, string> map;
        for (int i = 0; i < n; i++) {
            map[i] = to_string(i);
        }
    }
    ```

=== "Java"

    ```java title="space_complexity.java"
    /* 线性阶 */
    void linear(int n) {
        // 长度为 n 的数组占用 O(n) 空间
        int[] nums = new int[n];
        // 长度为 n 的列表占用 O(n) 空间
        List<ListNode> nodes = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            nodes.add(new ListNode(i));
        }
        // 长度为 n 的哈希表占用 O(n) 空间
        Map<Integer, String> map = new HashMap<>();
        for (int i = 0; i < n; i++) {
            map.put(i, String.valueOf(i));
        }
    }
    ```

=== "C#"

    ```csharp title="space_complexity.cs"
    /* 线性阶 */
    void Linear(int n) {
        // 长度为 n 的数组占用 O(n) 空间
        int[] nums = new int[n];
        // 长度为 n 的列表占用 O(n) 空间
        List<ListNode> nodes = [];
        for (int i = 0; i < n; i++) {
            nodes.Add(new ListNode(i));
        }
        // 长度为 n 的哈希表占用 O(n) 空间
        Dictionary<int, string> map = [];
        for (int i = 0; i < n; i++) {
            map.Add(i, i.ToString());
        }
    }
    ```

=== "Go"

    ```go title="space_complexity.go"
    /* 线性阶 */
    func spaceLinear(n int) {
        // 长度为 n 的数组占用 O(n) 空间
        _ = make([]int, n)
        // 长度为 n 的列表占用 O(n) 空间
        var nodes []*node
        for i := 0; i < n; i++ {
            nodes = append(nodes, newNode(i))
        }
        // 长度为 n 的哈希表占用 O(n) 空间
        m := make(map[int]string, n)
        for i := 0; i < n; i++ {
            m[i] = strconv.Itoa(i)
        }
    }
    ```

=== "Swift"

    ```swift title="space_complexity.swift"
    /* 线性阶 */
    func linear(n: Int) {
        // 长度为 n 的数组占用 O(n) 空间
        let nums = Array(repeating: 0, count: n)
        // 长度为 n 的列表占用 O(n) 空间
        let nodes = (0 ..< n).map { ListNode(x: $0) }
        // 长度为 n 的哈希表占用 O(n) 空间
        let map = Dictionary(uniqueKeysWithValues: (0 ..< n).map { ($0, "\($0)") })
    }
    ```

=== "JS"

    ```javascript title="space_complexity.js"
    /* 线性阶 */
    function linear(n) {
        // 长度为 n 的数组占用 O(n) 空间
        const nums = new Array(n);
        // 长度为 n 的列表占用 O(n) 空间
        const nodes = [];
        for (let i = 0; i < n; i++) {
            nodes.push(new ListNode(i));
        }
        // 长度为 n 的哈希表占用 O(n) 空间
        const map = new Map();
        for (let i = 0; i < n; i++) {
            map.set(i, i.toString());
        }
    }
    ```

=== "TS"

    ```typescript title="space_complexity.ts"
    /* 线性阶 */
    function linear(n: number): void {
        // 长度为 n 的数组占用 O(n) 空间
        const nums = new Array(n);
        // 长度为 n 的列表占用 O(n) 空间
        const nodes: ListNode[] = [];
        for (let i = 0; i < n; i++) {
            nodes.push(new ListNode(i));
        }
        // 长度为 n 的哈希表占用 O(n) 空间
        const map = new Map();
        for (let i = 0; i < n; i++) {
            map.set(i, i.toString());
        }
    }
    ```

=== "Dart"

    ```dart title="space_complexity.dart"
    /* 线性阶 */
    void linear(int n) {
      // 长度为 n 的数组占用 O(n) 空间
      List<int> nums = List.filled(n, 0);
      // 长度为 n 的列表占用 O(n) 空间
      List<ListNode> nodes = [];
      for (var i = 0; i < n; i++) {
        nodes.add(ListNode(i));
      }
      // 长度为 n 的哈希表占用 O(n) 空间
      Map<int, String> map = HashMap();
      for (var i = 0; i < n; i++) {
        map.putIfAbsent(i, () => i.toString());
      }
    }
    ```

=== "Rust"

    ```rust title="space_complexity.rs"
    /* 线性阶 */
    #[allow(unused)]
    fn linear(n: i32) {
        // 长度为 n 的数组占用 O(n) 空间
        let mut nums = vec![0; n as usize];
        // 长度为 n 的列表占用 O(n) 空间
        let mut nodes = Vec::new();
        for i in 0..n {
            nodes.push(ListNode::new(i))
        }
        // 长度为 n 的哈希表占用 O(n) 空间
        let mut map = HashMap::new();
        for i in 0..n {
            map.insert(i, i.to_string());
        }
    }
    ```

=== "C"

    ```c title="space_complexity.c"
    /* 哈希表 */
    typedef struct {
        int key;
        int val;
        UT_hash_handle hh; // 基于 uthash.h 实现
    } HashTable;

    /* 线性阶 */
    void linear(int n) {
        // 长度为 n 的数组占用 O(n) 空间
        int *nums = malloc(sizeof(int) * n);
        free(nums);

        // 长度为 n 的列表占用 O(n) 空间
        ListNode **nodes = malloc(sizeof(ListNode *) * n);
        for (int i = 0; i < n; i++) {
            nodes[i] = newListNode(i);
        }
        // 内存释放
        for (int i = 0; i < n; i++) {
            free(nodes[i]);
        }
        free(nodes);

        // 长度为 n 的哈希表占用 O(n) 空间
        HashTable *h = NULL;
        for (int i = 0; i < n; i++) {
            HashTable *tmp = malloc(sizeof(HashTable));
            tmp->key = i;
            tmp->val = i;
            HASH_ADD_INT(h, key, tmp);
        }

        // 内存释放
        HashTable *curr, *tmp;
        HASH_ITER(hh, h, curr, tmp) {
            HASH_DEL(h, curr);
            free(curr);
        }
    }
    ```

=== "Zig"

    ```zig title="space_complexity.zig"
    // 线性阶
    fn linear(comptime n: i32) !void {
        // 长度为 n 的数组占用 O(n) 空间
        var nums = [_]i32{0}**n;
        // 长度为 n 的列表占用 O(n) 空间
        var nodes = std.ArrayList(i32).init(std.heap.page_allocator);
        defer nodes.deinit();
        var i: i32 = 0;
        while (i < n) : (i += 1) {
            try nodes.append(i);
        }
        // 长度为 n 的哈希表占用 O(n) 空间
        var map = std.AutoArrayHashMap(i32, []const u8).init(std.heap.page_allocator);
        defer map.deinit();
        var j: i32 = 0;
        while (j < n) : (j += 1) {
            const string = try std.fmt.allocPrint(std.heap.page_allocator, "{d}", .{j});
            defer std.heap.page_allocator.free(string);
            try map.put(i, string);
        }
        _ = nums;
    }
    ```

As shown in the Figure 2-17 , the depth of recursion for this function is $n$, which means that there are $n$ unreturned `linear_recur()` functions at the same time, using $O(n)$ size stack frame space:

=== "Python"

    ```python title="space_complexity.py"
    def linear_recur(n: int):
        """线性阶（递归实现）"""
        print("递归 n =", n)
        if n == 1:
            return
        linear_recur(n - 1)
    ```

=== "C++"

    ```cpp title="space_complexity.cpp"
    /* 线性阶（递归实现） */
    void linearRecur(int n) {
        cout << "递归 n = " << n << endl;
        if (n == 1)
            return;
        linearRecur(n - 1);
    }
    ```

=== "Java"

    ```java title="space_complexity.java"
    /* 线性阶（递归实现） */
    void linearRecur(int n) {
        System.out.println("递归 n = " + n);
        if (n == 1)
            return;
        linearRecur(n - 1);
    }
    ```

=== "C#"

    ```csharp title="space_complexity.cs"
    /* 线性阶（递归实现） */
    void LinearRecur(int n) {
        Console.WriteLine("递归 n = " + n);
        if (n == 1) return;
        LinearRecur(n - 1);
    }
    ```

=== "Go"

    ```go title="space_complexity.go"
    /* 线性阶（递归实现） */
    func spaceLinearRecur(n int) {
        fmt.Println("递归 n =", n)
        if n == 1 {
            return
        }
        spaceLinearRecur(n - 1)
    }
    ```

=== "Swift"

    ```swift title="space_complexity.swift"
    /* 线性阶（递归实现） */
    func linearRecur(n: Int) {
        print("递归 n = \(n)")
        if n == 1 {
            return
        }
        linearRecur(n: n - 1)
    }
    ```

=== "JS"

    ```javascript title="space_complexity.js"
    /* 线性阶（递归实现） */
    function linearRecur(n) {
        console.log(`递归 n = ${n}`);
        if (n === 1) return;
        linearRecur(n - 1);
    }
    ```

=== "TS"

    ```typescript title="space_complexity.ts"
    /* 线性阶（递归实现） */
    function linearRecur(n: number): void {
        console.log(`递归 n = ${n}`);
        if (n === 1) return;
        linearRecur(n - 1);
    }
    ```

=== "Dart"

    ```dart title="space_complexity.dart"
    /* 线性阶（递归实现） */
    void linearRecur(int n) {
      print('递归 n = $n');
      if (n == 1) return;
      linearRecur(n - 1);
    }
    ```

=== "Rust"

    ```rust title="space_complexity.rs"
    /* 线性阶（递归实现） */
    fn linear_recur(n: i32) {
        println!("递归 n = {}", n);
        if n == 1 {return};
        linear_recur(n - 1);
    }
    ```

=== "C"

    ```c title="space_complexity.c"
    /* 线性阶（递归实现） */
    void linearRecur(int n) {
        printf("递归 n = %d\r\n", n);
        if (n == 1)
            return;
        linearRecur(n - 1);
    }
    ```

=== "Zig"

    ```zig title="space_complexity.zig"
    // 线性阶（递归实现）
    fn linearRecur(comptime n: i32) void {
        std.debug.print("递归 n = {}\n", .{n});
        if (n == 1) return;
        linearRecur(n - 1);
    }
    ```

![Linear order space complexity generated by recursion function](space_complexity.assets/space_complexity_recursive_linear.png){ class="animation-figure" }

<p align="center"> Figure 2-17 &nbsp; Linear order space complexity generated by recursion function </p>

### 3. &nbsp; Quadratic Order $O(N^2)$

Quadratic order is common in matrices and graphs, where the number of elements is in a square relationship with $n$:

=== "Python"

    ```python title="space_complexity.py"
    def quadratic(n: int):
        """平方阶"""
        # 二维列表占用 O(n^2) 空间
        num_matrix = [[0] * n for _ in range(n)]
    ```

=== "C++"

    ```cpp title="space_complexity.cpp"
    /* 平方阶 */
    void quadratic(int n) {
        // 二维列表占用 O(n^2) 空间
        vector<vector<int>> numMatrix;
        for (int i = 0; i < n; i++) {
            vector<int> tmp;
            for (int j = 0; j < n; j++) {
                tmp.push_back(0);
            }
            numMatrix.push_back(tmp);
        }
    }
    ```

=== "Java"

    ```java title="space_complexity.java"
    /* 平方阶 */
    void quadratic(int n) {
        // 矩阵占用 O(n^2) 空间
        int[][] numMatrix = new int[n][n];
        // 二维列表占用 O(n^2) 空间
        List<List<Integer>> numList = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            List<Integer> tmp = new ArrayList<>();
            for (int j = 0; j < n; j++) {
                tmp.add(0);
            }
            numList.add(tmp);
        }
    }
    ```

=== "C#"

    ```csharp title="space_complexity.cs"
    /* 平方阶 */
    void Quadratic(int n) {
        // 矩阵占用 O(n^2) 空间
        int[,] numMatrix = new int[n, n];
        // 二维列表占用 O(n^2) 空间
        List<List<int>> numList = [];
        for (int i = 0; i < n; i++) {
            List<int> tmp = [];
            for (int j = 0; j < n; j++) {
                tmp.Add(0);
            }
            numList.Add(tmp);
        }
    }
    ```

=== "Go"

    ```go title="space_complexity.go"
    /* 平方阶 */
    func spaceQuadratic(n int) {
        // 矩阵占用 O(n^2) 空间
        numMatrix := make([][]int, n)
        for i := 0; i < n; i++ {
            numMatrix[i] = make([]int, n)
        }
    }
    ```

=== "Swift"

    ```swift title="space_complexity.swift"
    /* 平方阶 */
    func quadratic(n: Int) {
        // 二维列表占用 O(n^2) 空间
        let numList = Array(repeating: Array(repeating: 0, count: n), count: n)
    }
    ```

=== "JS"

    ```javascript title="space_complexity.js"
    /* 平方阶 */
    function quadratic(n) {
        // 矩阵占用 O(n^2) 空间
        const numMatrix = Array(n)
            .fill(null)
            .map(() => Array(n).fill(null));
        // 二维列表占用 O(n^2) 空间
        const numList = [];
        for (let i = 0; i < n; i++) {
            const tmp = [];
            for (let j = 0; j < n; j++) {
                tmp.push(0);
            }
            numList.push(tmp);
        }
    }
    ```

=== "TS"

    ```typescript title="space_complexity.ts"
    /* 平方阶 */
    function quadratic(n: number): void {
        // 矩阵占用 O(n^2) 空间
        const numMatrix = Array(n)
            .fill(null)
            .map(() => Array(n).fill(null));
        // 二维列表占用 O(n^2) 空间
        const numList = [];
        for (let i = 0; i < n; i++) {
            const tmp = [];
            for (let j = 0; j < n; j++) {
                tmp.push(0);
            }
            numList.push(tmp);
        }
    }
    ```

=== "Dart"

    ```dart title="space_complexity.dart"
    /* 平方阶 */
    void quadratic(int n) {
      // 矩阵占用 O(n^2) 空间
      List<List<int>> numMatrix = List.generate(n, (_) => List.filled(n, 0));
      // 二维列表占用 O(n^2) 空间
      List<List<int>> numList = [];
      for (var i = 0; i < n; i++) {
        List<int> tmp = [];
        for (int j = 0; j < n; j++) {
          tmp.add(0);
        }
        numList.add(tmp);
      }
    }
    ```

=== "Rust"

    ```rust title="space_complexity.rs"
    /* 平方阶 */
    #[allow(unused)]
    fn quadratic(n: i32) {
        // 矩阵占用 O(n^2) 空间
        let num_matrix = vec![vec![0; n as usize]; n as usize];
        // 二维列表占用 O(n^2) 空间
        let mut num_list = Vec::new();
        for i in 0..n {
            let mut tmp = Vec::new();
            for j in 0..n {
                tmp.push(0);
            }
            num_list.push(tmp);
        }
    }
    ```

=== "C"

    ```c title="space_complexity.c"
    /* 平方阶 */
    void quadratic(int n) {
        // 二维列表占用 O(n^2) 空间
        int **numMatrix = malloc(sizeof(int *) * n);
        for (int i = 0; i < n; i++) {
            int *tmp = malloc(sizeof(int) * n);
            for (int j = 0; j < n; j++) {
                tmp[j] = 0;
            }
            numMatrix[i] = tmp;
        }

        // 内存释放
        for (int i = 0; i < n; i++) {
            free(numMatrix[i]);
        }
        free(numMatrix);
    }
    ```

=== "Zig"

    ```zig title="space_complexity.zig"
    // 平方阶
    fn quadratic(n: i32) !void {
        // 二维列表占用 O(n^2) 空间
        var nodes = std.ArrayList(std.ArrayList(i32)).init(std.heap.page_allocator);
        defer nodes.deinit();
        var i: i32 = 0;
        while (i < n) : (i += 1) {
            var tmp = std.ArrayList(i32).init(std.heap.page_allocator);
            defer tmp.deinit();
            var j: i32 = 0;
            while (j < n) : (j += 1) {
                try tmp.append(0);
            }
            try nodes.append(tmp);
        }
    }
    ```

As shown in the Figure 2-18 , the recursion depth of this function is $n$, and an array is initialized in each recursion function with lengths $n$, $n-1$, $\dots$, $2$, $1$, and an average length of $n / 2$, thus occupying $O(n^2)$ space overall:

=== "Python"

    ```python title="space_complexity.py"
    def quadratic_recur(n: int) -> int:
        """平方阶（递归实现）"""
        if n <= 0:
            return 0
        # 数组 nums 长度为 n, n-1, ..., 2, 1
        nums = [0] * n
        return quadratic_recur(n - 1)
    ```

=== "C++"

    ```cpp title="space_complexity.cpp"
    /* 平方阶（递归实现） */
    int quadraticRecur(int n) {
        if (n <= 0)
            return 0;
        vector<int> nums(n);
        cout << "递归 n = " << n << " 中的 nums 长度 = " << nums.size() << endl;
        return quadraticRecur(n - 1);
    }
    ```

=== "Java"

    ```java title="space_complexity.java"
    /* 平方阶（递归实现） */
    int quadraticRecur(int n) {
        if (n <= 0)
            return 0;
        // 数组 nums 长度为 n, n-1, ..., 2, 1
        int[] nums = new int[n];
        System.out.println("递归 n = " + n + " 中的 nums 长度 = " + nums.length);
        return quadraticRecur(n - 1);
    }
    ```

=== "C#"

    ```csharp title="space_complexity.cs"
    /* 平方阶（递归实现） */
    int QuadraticRecur(int n) {
        if (n <= 0) return 0;
        int[] nums = new int[n];
        Console.WriteLine("递归 n = " + n + " 中的 nums 长度 = " + nums.Length);
        return QuadraticRecur(n - 1);
    }
    ```

=== "Go"

    ```go title="space_complexity.go"
    /* 平方阶（递归实现） */
    func spaceQuadraticRecur(n int) int {
        if n <= 0 {
            return 0
        }
        nums := make([]int, n)
        fmt.Printf("递归 n = %d 中的 nums 长度 = %d \n", n, len(nums))
        return spaceQuadraticRecur(n - 1)
    }
    ```

=== "Swift"

    ```swift title="space_complexity.swift"
    /* 平方阶（递归实现） */
    @discardableResult
    func quadraticRecur(n: Int) -> Int {
        if n <= 0 {
            return 0
        }
        // 数组 nums 长度为 n, n-1, ..., 2, 1
        let nums = Array(repeating: 0, count: n)
        print("递归 n = \(n) 中的 nums 长度 = \(nums.count)")
        return quadraticRecur(n: n - 1)
    }
    ```

=== "JS"

    ```javascript title="space_complexity.js"
    /* 平方阶（递归实现） */
    function quadraticRecur(n) {
        if (n <= 0) return 0;
        const nums = new Array(n);
        console.log(`递归 n = ${n} 中的 nums 长度 = ${nums.length}`);
        return quadraticRecur(n - 1);
    }
    ```

=== "TS"

    ```typescript title="space_complexity.ts"
    /* 平方阶（递归实现） */
    function quadraticRecur(n: number): number {
        if (n <= 0) return 0;
        const nums = new Array(n);
        console.log(`递归 n = ${n} 中的 nums 长度 = ${nums.length}`);
        return quadraticRecur(n - 1);
    }
    ```

=== "Dart"

    ```dart title="space_complexity.dart"
    /* 平方阶（递归实现） */
    int quadraticRecur(int n) {
      if (n <= 0) return 0;
      List<int> nums = List.filled(n, 0);
      print('递归 n = $n 中的 nums 长度 = ${nums.length}');
      return quadraticRecur(n - 1);
    }
    ```

=== "Rust"

    ```rust title="space_complexity.rs"
    /* 平方阶（递归实现） */
    fn quadratic_recur(n: i32) -> i32 {
        if n <= 0 {return 0};
        // 数组 nums 长度为 n, n-1, ..., 2, 1
        let nums = vec![0; n as usize];
        println!("递归 n = {} 中的 nums 长度 = {}", n, nums.len());
        return quadratic_recur(n - 1);
    }
    ```

=== "C"

    ```c title="space_complexity.c"
    /* 平方阶（递归实现） */
    int quadraticRecur(int n) {
        if (n <= 0)
            return 0;
        int *nums = malloc(sizeof(int) * n);
        printf("递归 n = %d 中的 nums 长度 = %d\r\n", n, n);
        int res = quadraticRecur(n - 1);
        free(nums);
        return res;
    }
    ```

=== "Zig"

    ```zig title="space_complexity.zig"
    // 平方阶（递归实现）
    fn quadraticRecur(comptime n: i32) i32 {
        if (n <= 0) return 0;
        var nums = [_]i32{0}**n;
        std.debug.print("递归 n = {} 中的 nums 长度 = {}\n", .{n, nums.len});
        return quadraticRecur(n - 1);
    }
    ```

![Square-order space complexity generated by the recursion function](space_complexity.assets/space_complexity_recursive_quadratic.png){ class="animation-figure" }

<p align="center"> Figure 2-18 &nbsp; Square-order space complexity generated by the recursion function </p>

### 4. &nbsp; Exponential Order $O(2^N)$

Exponential order is common in binary trees. Looking at the Figure 2-19 , a "full binary tree" of degree $n$ has $2^n - 1$ nodes, occupying $O(2^n)$ space:

=== "Python"

    ```python title="space_complexity.py"
    def build_tree(n: int) -> TreeNode | None:
        """指数阶（建立满二叉树）"""
        if n == 0:
            return None
        root = TreeNode(0)
        root.left = build_tree(n - 1)
        root.right = build_tree(n - 1)
        return root
    ```

=== "C++"

    ```cpp title="space_complexity.cpp"
    /* 指数阶（建立满二叉树） */
    TreeNode *buildTree(int n) {
        if (n == 0)
            return nullptr;
        TreeNode *root = new TreeNode(0);
        root->left = buildTree(n - 1);
        root->right = buildTree(n - 1);
        return root;
    }
    ```

=== "Java"

    ```java title="space_complexity.java"
    /* 指数阶（建立满二叉树） */
    TreeNode buildTree(int n) {
        if (n == 0)
            return null;
        TreeNode root = new TreeNode(0);
        root.left = buildTree(n - 1);
        root.right = buildTree(n - 1);
        return root;
    }
    ```

=== "C#"

    ```csharp title="space_complexity.cs"
    /* 指数阶（建立满二叉树） */
    TreeNode? BuildTree(int n) {
        if (n == 0) return null;
        TreeNode root = new(0) {
            left = BuildTree(n - 1),
            right = BuildTree(n - 1)
        };
        return root;
    }
    ```

=== "Go"

    ```go title="space_complexity.go"
    /* 指数阶（建立满二叉树） */
    func buildTree(n int) *treeNode {
        if n == 0 {
            return nil
        }
        root := newTreeNode(0)
        root.left = buildTree(n - 1)
        root.right = buildTree(n - 1)
        return root
    }
    ```

=== "Swift"

    ```swift title="space_complexity.swift"
    /* 指数阶（建立满二叉树） */
    func buildTree(n: Int) -> TreeNode? {
        if n == 0 {
            return nil
        }
        let root = TreeNode(x: 0)
        root.left = buildTree(n: n - 1)
        root.right = buildTree(n: n - 1)
        return root
    }
    ```

=== "JS"

    ```javascript title="space_complexity.js"
    /* 指数阶（建立满二叉树） */
    function buildTree(n) {
        if (n === 0) return null;
        const root = new TreeNode(0);
        root.left = buildTree(n - 1);
        root.right = buildTree(n - 1);
        return root;
    }
    ```

=== "TS"

    ```typescript title="space_complexity.ts"
    /* 指数阶（建立满二叉树） */
    function buildTree(n: number): TreeNode | null {
        if (n === 0) return null;
        const root = new TreeNode(0);
        root.left = buildTree(n - 1);
        root.right = buildTree(n - 1);
        return root;
    }
    ```

=== "Dart"

    ```dart title="space_complexity.dart"
    /* 指数阶（建立满二叉树） */
    TreeNode? buildTree(int n) {
      if (n == 0) return null;
      TreeNode root = TreeNode(0);
      root.left = buildTree(n - 1);
      root.right = buildTree(n - 1);
      return root;
    }
    ```

=== "Rust"

    ```rust title="space_complexity.rs"
    /* 指数阶（建立满二叉树） */
    fn build_tree(n: i32) -> Option<Rc<RefCell<TreeNode>>> {
        if n == 0 {return None};
        let root = TreeNode::new(0);
        root.borrow_mut().left = build_tree(n - 1);
        root.borrow_mut().right = build_tree(n - 1);
        return Some(root);
    }
    ```

=== "C"

    ```c title="space_complexity.c"
    /* 指数阶（建立满二叉树） */
    TreeNode *buildTree(int n) {
        if (n == 0)
            return NULL;
        TreeNode *root = newTreeNode(0);
        root->left = buildTree(n - 1);
        root->right = buildTree(n - 1);
        return root;
    }
    ```

=== "Zig"

    ```zig title="space_complexity.zig"
    // 指数阶（建立满二叉树）
    fn buildTree(mem_allocator: std.mem.Allocator, n: i32) !?*inc.TreeNode(i32) {
        if (n == 0) return null;
        const root = try mem_allocator.create(inc.TreeNode(i32));
        root.init(0);
        root.left = try buildTree(mem_allocator, n - 1);
        root.right = try buildTree(mem_allocator, n - 1);
        return root;
    }
    ```

![Exponential order space complexity generated by a full binary tree](space_complexity.assets/space_complexity_exponential.png){ class="animation-figure" }

<p align="center"> Figure 2-19 &nbsp; Exponential order space complexity generated by a full binary tree </p>

### 5. &nbsp; Logarithmic Order $O(\Log N)$

Logarithmic order is commonly used in divide and conquer algorithms. For example, in a merge sort, given an array of length $n$ as the input, each round of recursion divides the array in half from its midpoint to form a recursion tree of height $\log n$, using $O(\log n)$ stack frame space.

Another example is to convert a number into a string. Given a positive integer $n$ with a digit count of $\log_{10} n + 1$, the corresponding string length is $\log_{10} n + 1$. Therefore, the space complexity is $O(\log_{10} n + 1) = O(\log n)$.

## 2.4.4 &nbsp; Weighing Time And Space

Ideally, we would like to optimize both the time complexity and the space complexity of an algorithm. However, in reality, simultaneously optimizing time and space complexity is often challenging.

**Reducing time complexity usually comes at the expense of increasing space complexity, and vice versa**. The approach of sacrificing memory space to improve algorithm speed is known as "trading space for time", while the opposite is called "trading time for space".

The choice between these approaches depends on which aspect we prioritize. In most cases, time is more valuable than space, so "trading space for time" is usually the more common strategy. Of course, in situations with large data volumes, controlling space complexity is also crucial.