feat(src): add 15-threeSum

Author: kwu130

src/15-threeSum.cpp

@@ -0,0 +1,59 @@
+//
+// Created by 1412 on 2025/10/12.
+//
+
+#include <bits/stdc++.h>
+
+using namespace std;
+
+class Solution {
+public:
+    vector<vector<int>> threeSum(vector<int>& nums) {
+        if (nums.size() < 3) {
+            return {};
+        }
+        vector<vector<int>> res;
+        ranges::sort(nums);
+        for (int i = 0; i < nums.size() - 2; i++) {
+            if (i > 0 && nums[i] == nums[i - 1]) {
+                continue;
+            }
+            int left = i + 1, right = nums.size() - 1;
+            while (left < right) {
+                int sum = nums[i] + nums[left] + nums[right];
+                if (sum == 0) {
+                    res.push_back({nums[i], nums[left++], nums[right--]});
+                    while (left < right && nums[left] == nums[left - 1]) {
+                        left++;
+                    }
+                    while (left < right && nums[right] == nums[right + 1]) {
+                        right--;
+                    }
+                } else if (sum < 0) {
+                    left++;
+                } else {
+                    right--;
+                }
+            }
+        }
+        return res;
+    }
+};
+
+void test() {
+    Solution s;
+    vector<int> nums = {-1, 0, 1, 2, -1, -4};
+    vector<vector<int>> expected = {{{-1, -1, 2}, {-1, 0, 1}}};
+    assert(s.threeSum(nums) == expected);
+    nums = {0, 1, 1};
+    expected = {};
+    assert(s.threeSum(nums) == expected);
+    nums = {0, 0, 0};
+    expected = {{0, 0, 0}};
+    assert(s.threeSum(nums) == expected);
+}
+
+int main() {
+    test();
+    return 0;
+}