diff --git a/codes/python/chapter_dynamic_programming/knapsack.py b/codes/python/chapter_dynamic_programming/knapsack.py new file mode 100644 index 000000000..9f8d3cff4 --- /dev/null +++ b/codes/python/chapter_dynamic_programming/knapsack.py @@ -0,0 +1,99 @@ +""" +File: knapsack.py +Created Time: 2023-07-03 +Author: Krahets (krahets@163.com) +""" + + +def knapsack_dfs(wgt, val, i, c): + """0-1 背包:暴力搜索""" + # 若已选完所有物品或背包无容量,则返回价值 0 + if i == 0 or c == 0: + return 0 + # 若超过背包容量,则只能不放入背包 + if wgt[i - 1] > c: + return knapsack_dfs(wgt, val, i - 1, c) + # 计算不放入和放入物品 i 的最大价值 + no = knapsack_dfs(wgt, val, i - 1, c) + yes = knapsack_dfs(wgt, val, i - 1, c - wgt[i - 1]) + val[i - 1] + # 返回两种方案中价值更大的那一个 + return max(no, yes) + + +def knapsack_dfs_mem(wgt, val, mem, i, c): + """0-1 背包:记忆化搜索""" + # 若已选完所有物品或背包无容量,则返回价值 0 + if i == 0 or c == 0: + return 0 + # 若已有记录,则直接返回 + if mem[i][c] != -1: + return mem[i][c] + # 若超过背包容量,则只能不放入背包 + if wgt[i - 1] > c: + return knapsack_dfs_mem(wgt, val, mem, i - 1, c) + # 计算不放入和放入物品 i 的最大价值 + no = knapsack_dfs_mem(wgt, val, mem, i - 1, c) + yes = knapsack_dfs_mem(wgt, val, mem, i - 1, c - wgt[i - 1]) + val[i - 1] + # 记录并返回两种方案中价值更大的那一个 + mem[i][c] = max(no, yes) + return mem[i][c] + + +def knapsack_dp(wgt, val, cap): + """0-1 背包:动态规划""" + n = len(wgt) + # 初始化 dp 列表 + dp = [[0] * (cap + 1) for _ in range(n + 1)] + # 状态转移 + for i in range(1, n + 1): + for c in range(1, cap + 1): + if wgt[i - 1] > c: + # 若超过背包容量,则不选物品 i + dp[i][c] = dp[i - 1][c] + else: + # 不选和选物品 i 这两种方案的较大值 + dp[i][c] = max(dp[i - 1][c - wgt[i - 1]] + val[i - 1], dp[i - 1][c]) + return dp[n][cap] + + +def knapsack_dp_comp(wgt, val, cap): + """0-1 背包:状态压缩后的动态规划""" + n = len(wgt) + # 初始化 dp 列表 + dp = [0] * (cap + 1) + # 状态转移 + for i in range(1, n + 1): + # 倒序遍历 + for c in range(cap, 0, -1): + if wgt[i - 1] > c: + # 若超过背包容量,则不选物品 i + dp[c] = dp[c] + else: + # 不选和选物品 i 这两种方案的较大值 + dp[c] = max(dp[c - wgt[i - 1]] + val[i - 1], dp[c]) + return dp[cap] + + +"""Driver Code""" +if __name__ == "__main__": + wgt = [10, 20, 30, 40, 50] + val = [60, 100, 120, 160, 200] + cap = 50 + n = len(wgt) + + # 暴力搜索 + res = knapsack_dfs(wgt, val, n, cap) + print(res) + + # 记忆化搜索 + mem = [[-1] * (cap + 1) for _ in range(n + 1)] + res = knapsack_dfs_mem(wgt, val, mem, n, cap) + print(res) + + # 动态规划 + res = knapsack_dp(wgt, val, cap) + print(res) + + # 状态压缩后的动态规划 + res = knapsack_dp_comp(wgt, val, cap) + print(res) diff --git a/docs/chapter_dynamic_programming/knapsack_problem.assets/knapsack_dfs.png b/docs/chapter_dynamic_programming/knapsack_problem.assets/knapsack_dfs.png new file mode 100644 index 000000000..fa1e10926 Binary files /dev/null and b/docs/chapter_dynamic_programming/knapsack_problem.assets/knapsack_dfs.png differ diff --git a/docs/chapter_dynamic_programming/knapsack_problem.assets/knapsack_dfs_mem.png b/docs/chapter_dynamic_programming/knapsack_problem.assets/knapsack_dfs_mem.png new file mode 100644 index 000000000..6b55cb990 Binary files /dev/null and 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Binary files /dev/null and b/docs/chapter_dynamic_programming/knapsack_problem.assets/knapsack_dp_step9.png differ diff --git a/docs/chapter_dynamic_programming/knapsack_problem.assets/knapsack_example.png b/docs/chapter_dynamic_programming/knapsack_problem.assets/knapsack_example.png new file mode 100644 index 000000000..292780df4 Binary files /dev/null and b/docs/chapter_dynamic_programming/knapsack_problem.assets/knapsack_example.png differ diff --git a/docs/chapter_dynamic_programming/knapsack_problem.md b/docs/chapter_dynamic_programming/knapsack_problem.md new file mode 100644 index 000000000..c8151dbfe --- /dev/null +++ b/docs/chapter_dynamic_programming/knapsack_problem.md @@ -0,0 +1,393 @@ +# 0-1 背包问题 + +背包问题是学习动态规划的一个非常好的入门题目,其涉及到“选择与不选择”和“限制条件下的最优化”等问题,是动态规划中最常见的问题形式。 + +背包问题具有很多变种,例如 0-1 背包问题、完全背包问题、多重背包问题等。在本节中,我们先来学习最简单的 0-1 背包问题。 + +!!! question + + 给定 $n$ 个物品,第 $i$ 个物品的重量为 $wgt[i-1]$ 、价值为 $val[i-1]$ ,现在有个容量为 $cap$ 的背包,请求解在不超过背包容量下背包中物品的最大价值。 + + 请注意,物品编号 $i$ 从 $1$ 开始计数,但数组索引从 $0$ 开始计数,因此物品 $i$ 对应重量 $wgt[i-1]$ 和价值 $val[i-1]$ 。 + +下图给出了一个 0-1 背包的示例数据,背包内的最大价值为 $220$ 。 + +![0-1 背包的示例数据](knapsack_problem.assets/knapsack_example.png) + +接下来,我们仍然先从回溯角度入手,先给出暴力搜索解法;再引入记忆化处理,得到记忆化搜索和动态规划解法。 + +## 方法一:暴力搜索 + +0-1 背包问题是一道典型的“选或不选”的问题,0 代表不选、1 代表选。我们可以将 0-1 背包看作是一个由 $n$ 轮决策组成的搜索过程,对于每个物体都有不放入和放入两种决策。不放入背包,背包容量不变;放入背包,背包容量减小。由此可得: + +- **状态包括物品编号 $i$ 和背包容量 $c$**,记为 $[i, c]$ 。 +- 状态 $[i, c]$ 对应子问题“**前 $i$ 个物品在容量为 $c$ 背包中的最大价值**”,解记为 $dp[i, c]$ 。 + +当我们做出物品 $i$ 的决策后,剩余的是前 $i-1$ 个物品的子问题,因此状态转移分为两种: + +- **不放入物品 $i$** :背包容量不变,状态转移至 $[i-1, c]$ ; +- **放入物品 $i$** :背包容量减小 $wgt[i-1]$ ,价值增加 $val[i-1]$ ,状态转移至 $[i-1, c-wgt[i-1]]$ ; + +上述的状态转移向我们展示了本题的「最优子结构」:**最大价值 $dp[i, c]$ 等于不放入物品 $i$ 和放入物品 $i$ 两种方案中的价值更大的那一个**。由此可推出状态转移方程: + +$$ +dp[i, c] = \max(dp[i-1, c], dp[i-1, c - wgt[i-1]] + val[i-1]) +$$ + +以下是暴力搜索的实现代码,其中包含以下要素: + +- **递归参数**:状态 $[i, c]$ ;**返回值**:子问题的解 $dp[i, c]$ 。 +- **终止条件**:当已完成 $n$ 轮决策或背包无剩余容量为时,终止递归并返回价值 $0$ 。 +- **剪枝**:若当前物品重量 $wgt[i - 1]$ 超出剩余背包容量 $c$ ,则只能选择不放入背包。 + +=== "Java" + + ```java title="knapsack.java" + [class]{knapsack}-[func]{knapsackDFS} + ``` + +=== "C++" + + ```cpp title="knapsack.cpp" + [class]{}-[func]{knapsackDFS} + ``` + +=== "Python" + + ```python title="knapsack.py" + [class]{}-[func]{knapsack_dfs} + ``` + +=== "Go" + + ```go title="knapsack.go" + [class]{}-[func]{knapsackDFS} + ``` + +=== "JavaScript" + + ```javascript title="knapsack.js" + [class]{}-[func]{knapsackDFS} + ``` + +=== "TypeScript" + + ```typescript title="knapsack.ts" + [class]{}-[func]{knapsackDFS} + ``` + +=== "C" + + ```c title="knapsack.c" + [class]{}-[func]{knapsackDFS} + ``` + +=== "C#" + + ```csharp title="knapsack.cs" + [class]{knapsack}-[func]{knapsackDFS} + ``` + +=== "Swift" + + ```swift title="knapsack.swift" + [class]{}-[func]{knapsackDFS} + ``` + +=== "Zig" + + ```zig title="knapsack.zig" + [class]{}-[func]{knapsackDFS} + ``` + +=== "Dart" + + ```dart title="knapsack.dart" + [class]{}-[func]{knapsackDFS} + ``` + +如下图所示,由于每个物品都会产生不选和选两条搜索分支,因此最差时间复杂度为 $O(2^n)$ 。 + +观察递归树,容易发现其中存在一些「重叠子问题」。而当物品较多、背包容量较大,尤其是当相同重量的物品较多时,重叠子问题的数量将会大幅增多。 + +![0-1 背包的暴力搜索递归树](knapsack_problem.assets/knapsack_dfs.png) + +## 方法二:记忆化搜索 + +为了防止重复求解重叠子问题,我们借助一个记忆列表 `mem` 来记录子问题的解,其中 `mem[i][c]` 表示前 $i$ 个物品在容量为 $c$ 背包中的最大价值。当再次遇到相同子问题时,直接从 `mem` 中获取记录。 + +=== "Java" + + ```java title="knapsack.java" + [class]{knapsack}-[func]{knapsackDFSMem} + ``` + +=== "C++" + + ```cpp title="knapsack.cpp" + [class]{}-[func]{knapsackDFSMem} + ``` + +=== "Python" + + ```python title="knapsack.py" + [class]{}-[func]{knapsack_dfs_mem} + ``` + +=== "Go" + + ```go title="knapsack.go" + [class]{}-[func]{knapsackDFSMem} + ``` + +=== "JavaScript" + + ```javascript title="knapsack.js" + [class]{}-[func]{knapsackDFSMem} + ``` + +=== "TypeScript" + + ```typescript title="knapsack.ts" + [class]{}-[func]{knapsackDFSMem} + ``` + +=== "C" + + ```c title="knapsack.c" + [class]{}-[func]{knapsackDFSMem} + ``` + +=== "C#" + + ```csharp title="knapsack.cs" + [class]{knapsack}-[func]{knapsackDFSMem} + ``` + +=== "Swift" + + ```swift title="knapsack.swift" + [class]{}-[func]{knapsackDFSMem} + ``` + +=== "Zig" + + ```zig title="knapsack.zig" + [class]{}-[func]{knapsackDFSMem} + ``` + +=== "Dart" + + ```dart title="knapsack.dart" + [class]{}-[func]{knapsackDFSMem} + ``` + +引入记忆化之后,所有子问题最多只被计算一次,**因此时间复杂度取决于子问题数量**,也就是 $O(n \times cap)$ 。 + +![0-1 背包的记忆化搜索递归树](knapsack_problem.assets/knapsack_dfs_mem.png) + +## 方法三:动态规划 + +接下来就是体力活了,我们将“从顶至底”的记忆化搜索代码译写为“从底至顶”的动态规划代码。 + +=== "Java" + + ```java title="knapsack.java" + [class]{knapsack}-[func]{knapsackDP} + ``` + +=== "C++" + + ```cpp title="knapsack.cpp" + [class]{}-[func]{knapsackDP} + ``` + +=== "Python" + + ```python title="knapsack.py" + [class]{}-[func]{knapsack_dp} + ``` + +=== "Go" + + ```go title="knapsack.go" + [class]{}-[func]{knapsackDP} + ``` + +=== "JavaScript" + + ```javascript title="knapsack.js" + [class]{}-[func]{knapsackDP} + ``` + +=== "TypeScript" + + ```typescript title="knapsack.ts" + [class]{}-[func]{knapsackDP} + ``` + +=== "C" + + ```c title="knapsack.c" + [class]{}-[func]{knapsackDP} + ``` + +=== "C#" + + ```csharp title="knapsack.cs" + [class]{knapsack}-[func]{knapsackDP} + ``` + +=== "Swift" + + ```swift title="knapsack.swift" + [class]{}-[func]{knapsackDP} + ``` + +=== "Zig" + + ```zig title="knapsack.zig" + [class]{}-[func]{knapsackDP} + ``` + +=== "Dart" + + ```dart title="knapsack.dart" + [class]{}-[func]{knapsackDP} + ``` + +观察下图,动态规划的过程本质上就是填充 $dp$ 列表(矩阵)的过程,时间复杂度也为 $O(n \times cap)$ 。 + +=== "<1>" + ![0-1 背包的动态规划过程](knapsack_problem.assets/knapsack_dp_step1.png) + +=== "<2>" + ![knapsack_dp_step2](knapsack_problem.assets/knapsack_dp_step2.png) + +=== "<3>" + ![knapsack_dp_step3](knapsack_problem.assets/knapsack_dp_step3.png) + +=== "<4>" + ![knapsack_dp_step4](knapsack_problem.assets/knapsack_dp_step4.png) + +=== "<5>" + ![knapsack_dp_step5](knapsack_problem.assets/knapsack_dp_step5.png) + +=== "<6>" + ![knapsack_dp_step6](knapsack_problem.assets/knapsack_dp_step6.png) + +=== "<7>" + ![knapsack_dp_step7](knapsack_problem.assets/knapsack_dp_step7.png) + +=== "<8>" + ![knapsack_dp_step8](knapsack_problem.assets/knapsack_dp_step8.png) + +=== "<9>" + ![knapsack_dp_step9](knapsack_problem.assets/knapsack_dp_step9.png) + +=== "<10>" + ![knapsack_dp_step10](knapsack_problem.assets/knapsack_dp_step10.png) + +=== "<11>" + ![knapsack_dp_step11](knapsack_problem.assets/knapsack_dp_step11.png) + +=== "<12>" + ![knapsack_dp_step12](knapsack_problem.assets/knapsack_dp_step12.png) + +=== "<13>" + ![knapsack_dp_step13](knapsack_problem.assets/knapsack_dp_step13.png) + +=== "<14>" + ![knapsack_dp_step14](knapsack_problem.assets/knapsack_dp_step14.png) + +**接下来考虑状态压缩**。以上代码中的 $dp$ 矩阵占用 $O(n \times cap)$ 空间。由于每个状态都只与其上一行的状态有关,因此我们可以使用两个数组滚动前进,将空间复杂度从 $O(n^2)$ 将低至 $O(n)$ 。代码省略,有兴趣的同学可以自行实现。 + +那么,我们是否可以仅用一个数组实现状态压缩呢?观察可知,每个状态都是由左上方或正上方的格子转移过来的。假设只有一个数组,当遍历到第 $i$ 行时,该数组存储的仍然是第 $i-1$ 行的状态,为了避免左边区域的格子被覆盖,我们应采取倒序遍历,这样方可实现正确的状态转移。 + +以下动画展示了在单个数组下从第 $i=1$ 行转换至第 $i=2$ 行的过程。建议你思考一下正序遍历和倒序遍历的区别。 + +=== "<1>" + ![0-1 背包的状态压缩后的动态规划过程](knapsack_problem.assets/knapsack_dp_comp_step1.png) + +=== "<2>" + ![knapsack_dp_comp_step2](knapsack_problem.assets/knapsack_dp_comp_step2.png) + +=== "<3>" + ![knapsack_dp_comp_step3](knapsack_problem.assets/knapsack_dp_comp_step3.png) + +=== "<4>" + ![knapsack_dp_comp_step4](knapsack_problem.assets/knapsack_dp_comp_step4.png) + +=== "<5>" + ![knapsack_dp_comp_step5](knapsack_problem.assets/knapsack_dp_comp_step5.png) + +=== "<6>" + ![knapsack_dp_comp_step6](knapsack_problem.assets/knapsack_dp_comp_step6.png) + +如以下代码所示,我们仅需将 $dp$ 列表的第一维 $i$ 直接删除,并且将内循环修改为倒序遍历即可。 + +=== "Java" + + ```java title="knapsack.java" + [class]{knapsack}-[func]{knapsackDPComp} + ``` + +=== "C++" + + ```cpp title="knapsack.cpp" + [class]{}-[func]{knapsackDPComp} + ``` + +=== "Python" + + ```python title="knapsack.py" + [class]{}-[func]{knapsack_dp_comp} + ``` + +=== "Go" + + ```go title="knapsack.go" + [class]{}-[func]{knapsackDPComp} + ``` + +=== "JavaScript" + + ```javascript title="knapsack.js" + [class]{}-[func]{knapsackDPComp} + ``` + +=== "TypeScript" + + ```typescript title="knapsack.ts" + [class]{}-[func]{knapsackDPComp} + ``` + +=== "C" + + ```c title="knapsack.c" + [class]{}-[func]{knapsackDPComp} + ``` + +=== "C#" + + ```csharp title="knapsack.cs" + [class]{knapsack}-[func]{knapsackDPComp} + ``` + +=== "Swift" + + ```swift title="knapsack.swift" + [class]{}-[func]{knapsackDPComp} + ``` + +=== "Zig" + + ```zig title="knapsack.zig" + [class]{}-[func]{knapsackDPComp} + ``` + +=== "Dart" + + ```dart title="knapsack.dart" + [class]{}-[func]{knapsackDPComp} + ``` diff --git a/mkdocs.yml b/mkdocs.yml index 950a79680..cfcfca309 100644 --- a/mkdocs.yml +++ b/mkdocs.yml @@ -216,6 +216,7 @@ nav: - chapter_dynamic_programming/index.md - 13.1.   初探动态规划(New): chapter_dynamic_programming/intro_to_dynamic_programming.md - 13.2.   DP 问题特性(New): chapter_dynamic_programming/dp_problem_features.md + - 13.3.   0-1 背包问题(New): chapter_dynamic_programming/knapsack_problem.md - 14.     附录: - 14.1.   编程环境安装: chapter_appendix/installation.md - 14.2.   一起参与创作: chapter_appendix/contribution.md