add 3sum

Author: Danieldu

150/15.3-sum.cpp

@@ -0,0 +1,51 @@
+/*
+ * @lc app=leetcode id=15 lang=cpp
+ *
+ * [15] 3Sum
+ */
+
+// @lc code=start
+#include <bits/stdc++.h>
+using namespace std;
+
+class Solution {
+public:
+    vector<vector<int>> threeSum(vector<int>& nums) {
+        vector<vector<int>> res;
+        sort(nums.begin(), nums.end());
+        
+        int n = nums.size();
+        
+        for (int i = 0; i < n - 2; ++i) {
+
+            if (nums[i] > 0) break;
+            
+            if (i > 0 && nums[i] == nums[i - 1]) continue;
+            
+            int left = i + 1;
+            int right = n - 1;
+            
+            while (left < right) {
+                long long total = nums[i] + nums[left] + nums[right];
+                
+                if (total < 0) {
+                    left++;
+                } else if (total > 0) {
+                    right--;
+                } else {
+                    res.push_back({nums[i], nums[left], nums[right]});
+                    
+                    left++;
+                    while (left < right && nums[left] == nums[left - 1]) 
+                        left++;             
+                    right--;
+                    while (left < right && nums[right] == nums[right + 1]) 
+                        right--;
+                }
+            }
+        }
+        return res;
+    }
+};
+// @lc code=end
+