feat(src): add 40-combinationSum2

Author: kwu130

src/40-combinationSum2.cpp

@@ -0,0 +1,56 @@
+//
+// Created by 1412 on 2025/10/24.
+//
+
+#include <bits/stdc++.h>
+
+using namespace std;
+
+class Solution {
+public:
+    vector<vector<int>> combinationSum2(vector<int>& candidates, int target) {
+        vector<vector<int>> res;
+        ranges::sort(candidates);
+        dfs(candidates, target, 0, {}, res);
+        return res;
+    }
+
+private:
+    void dfs(vector<int>& candidates, int target, int index, vector<int>&& path, vector<vector<int>>& res) {
+        if (target == 0) {
+            res.emplace_back(path);
+            return;
+        }
+        for (int i = index; i < candidates.size(); i++) {
+            if (candidates[i] > target) break;
+            if (i > index && candidates[i] == candidates[i - 1]) continue;  // 剪枝 重点理解这个 i>index 的作用
+            path.emplace_back(candidates[i]);
+            dfs(candidates, target - candidates[i], i + 1, std::move(path), res);
+            path.pop_back();
+        }
+    }
+};
+
+void test() {
+    Solution s;
+    vector<int> candidates = {10, 1, 2, 7, 6, 1, 5};
+    vector<vector<int>> expected = {
+        {1, 1, 6},
+        {1, 2, 5},
+        {1, 7},
+        {2, 6}
+    };
+    assert(s.combinationSum2(candidates, 8) == expected);
+
+    candidates = {2, 5, 2, 1, 2};
+    expected = {
+        {1, 2, 2},
+        {5}
+    };
+    assert(s.combinationSum2(candidates, 5) == expected);
+}
+
+int main() {
+    test();
+    return 0;
+}