feat(csharp): add the section of iteration and recursion (#697)

codes/csharp/GlobalUsing.cs

@@ -1,2 +1,3 @@
 global using NUnit.Framework;
-global using hello_algo.utils;
+global using hello_algo.utils;
+global using System.Text;

codes/csharp/chapter_computational_complexity/iteration.cs

@@ -0,0 +1,77 @@
+/**
+* File: iteration.cs
+* Created Time: 2023-08-28
+* Author: hpstory (hpstory1024@163.com)
+*/
+
+namespace hello_algo.chapter_computational_complexity;
+
+public class iteration {
+    /* for 循环 */
+    public int forLoop(int n) {
+        int res = 0;
+        // 循环求和 1, 2, ..., n-1, n
+        for (int i = 1; i <= n; i++) {
+            res += i;
+        }
+        return res;
+    }
+
+    /* while 循环 */
+    public int whileLoop(int n) {
+        int res = 0;
+        int i = 1; // 初始化条件变量
+        // 循环求和 1, 2, ..., n-1, n
+        while (i <= n) {
+            res += i;
+            i += 1; // 更新条件变量
+        }
+        return res;
+    }
+
+    /* while 循环（两次更新） */
+    public int whileLoopII(int n) {
+        int res = 0;
+        int i = 1; // 初始化条件变量
+        // 循环求和 1, 2, 4, 5...
+        while (i <= n) {
+            res += i;
+            i += 1; // 更新条件变量
+            res += i;
+            i *= 2; // 更新条件变量
+        }
+        return res;
+    }
+
+    /* 双层 for 循环 */
+    public string nestedForLoop(int n) {
+        StringBuilder res = new StringBuilder();
+        // 循环 i = 1, 2, ..., n-1, n
+        for (int i = 1; i <= n; i++) {
+            // 循环 j = 1, 2, ..., n-1, n
+            for (int j = 1; j <= n; j++) {
+                res.Append($"({i}, {j}), ");
+            }
+        }
+        return res.ToString();
+    }
+
+    /* Driver Code */
+    [Test]
+    public void Test() {
+        int n = 5;
+        int res;
+
+        res = forLoop(n);
+        Console.WriteLine("\nfor 循环的求和结果 res = " + res);
+
+        res = whileLoop(n);
+        Console.WriteLine("\nwhile 循环的求和结果 res = " + res);
+
+        res = whileLoopII(n);
+        Console.WriteLine("\nwhile 循环（两次更新）求和结果 res = " + res);
+
+        string resStr = nestedForLoop(n);
+        Console.WriteLine("\n双层 for 循环的遍历结果 " + resStr);
+    }
+}

codes/csharp/chapter_computational_complexity/recursion.cs

@@ -0,0 +1,56 @@
+/**
+* File: recursion.cs
+* Created Time: 2023-08-28
+* Author: hpstory (hpstory1024@163.com)
+*/
+
+namespace hello_algo.chapter_computational_complexity;
+
+public class recursion {
+    /* 递归 */
+    public int recur(int n) {
+        // 终止条件
+        if (n == 1)
+            return 1;
+        // 递：递归调用
+        int res = recur(n - 1);
+        // 归：返回结果
+        return n + res;
+    }
+
+    /* 尾递归 */
+    public int tailRecur(int n, int res) {
+        // 终止条件
+        if (n == 0)
+            return res;
+        // 尾递归调用
+        return tailRecur(n - 1, res + n);
+    }
+
+    /* 斐波那契数列：递归 */
+    public int fib(int n) {
+        // 终止条件 f(1) = 0, f(2) = 1
+        if (n == 1 || n == 2)
+            return n - 1;
+        // 递归调用 f(n) = f(n-1) + f(n-2)
+        int res = fib(n - 1) + fib(n - 2);
+        // 返回结果 f(n)
+        return res;
+    }
+
+    /* Driver Code */
+    [Test]
+    public void Test() {
+        int n = 5;
+        int res;
+
+        res = recur(n);
+        Console.WriteLine("\n递归函数的求和结果 res = " + res);
+
+        res = tailRecur(n, 0);
+        Console.WriteLine("\n尾递归函数的求和结果 res = " + res);
+
+        res = fib(n);
+        Console.WriteLine("\n斐波那契数列的第 " + n + " 项为 " + res);
+    }
+}