From e825f1faeb459a61c258641f49b3b40c64ff05dd Mon Sep 17 00:00:00 2001
From: potrue <126231160+potrue@users.noreply.github.com>
Date: Wed, 20 Aug 2025 18:13:24 +0900
Subject: [PATCH] 39. Combination Sum

https://leetcode.com/problems/combination-sum/description/
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 39/39.md | 112 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 112 insertions(+)
 create mode 100644 39/39.md

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+## 何も見ずに解いてみる
+
+```cpp
+class Solution {
+public:
+    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
+        vector<vector<int>> result;
+        vector<int> initial_combination;
+        combinationSumHelper(candidates, initial_combination, target, result);
+        return result;
+    }
+private:
+    void combinationSumHelper(vector<int>& candidates, vector<int> combination, int target, vector<vector<int>>& result) {
+        if (target == 0) {
+            result.emplace_back(combination);
+            return;
+        }
+        if (candidates.empty()) {
+            return;
+        }
+        int num = candidates.back();
+        candidates.pop_back();
+        while (target >= 0) {
+            combinationSumHelper(candidates, combination, target, result);
+            combination.emplace_back(num);
+            target -= num;
+        }
+        candidates.emplace_back(num);
+    }
+};
+```
+
+## 他の人のコードを見てみる
+
+https://github.com/tokuhirat/LeetCode/pull/52/files  
+https://github.com/ryosuketc/leetcode_arai60/pull/41/files  
+https://github.com/Ryotaro25/leetcode_first60/pull/56/files  
+
+最初に書いたコードはcombinationが値渡しであったことで結構時間がかかっていたようです。  
+以下のようにするだけでleetcode上の実行時間が20msぐらいから1msぐらいになりました。
+
+```cpp
+class Solution {
+public:
+    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
+        vector<vector<int>> result;
+        vector<int> combination;
+        combinationSumHelper(candidates, combination, target, result);
+        return result;
+    }
+private:
+    void combinationSumHelper(vector<int>& candidates, vector<int>& combination, int target, vector<vector<int>>& result) {
+        if (target == 0) {
+            result.emplace_back(combination);
+            return;
+        }
+        if (candidates.empty()) {
+            return;
+        }
+        int num = candidates.back();
+        candidates.pop_back();
+        while (target >= 0) {
+            combinationSumHelper(candidates, combination, target, result);
+            combination.emplace_back(num);
+            target -= num;
+        }
+        candidates.emplace_back(num);
+        while (!combination.empty() && combination.back() == num) {
+            combination.pop_back();
+        }
+    }
+};
+```
+
+大きい数字から見ていったほうが若干高速ではありそうですが、出力の形がきれいじゃなくなってしまいますね。
+
+## 最終コード
+
+```cpp
+class Solution {
+public:
+    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
+        vector<vector<int>> result;
+        vector<int> initial_combination;
+        combinationSumHelper(candidates, initial_combination, 0, target, result);
+        return result;
+    }
+private:
+    void combinationSumHelper(
+        const vector<int>& candidates,
+        vector<int>& combination,
+        int processed_until,
+        int target,
+        vector<vector<int>>& result
+    ) {
+        if (target < 0) {
+            return;
+        }
+        if (target == 0) {
+            result.emplace_back(combination);
+            return;
+        }
+        for (int i = processed_until; i < candidates.size(); ++i) {
+            combination.emplace_back(candidates[i]);
+            combinationSumHelper(candidates, combination, i, target - candidates[i], result);
+            combination.pop_back();
+        }
+    }
+};
+```
+
+最後のiという変数名にはもう少し意味を持たせたいと思ったのですが、あまり簡潔な変数名が思いつかず・・・