Added 3Sum.cpp in Leetcode

Author: unsaid-u

Leetcode/ARRAY/3Sum.cpp

@@ -0,0 +1,43 @@
+
+/*
+	15. 3Sum
+	solution C++
+	O(N-square)
+*/
+
+
+class Solution {
+public:
+    vector<vector<int>> threeSum(vector<int>& nums) {
+        set<vector<int>> ans;
+        int n= nums.size();
+        if(n <3)
+            return {};
+        
+        sort(nums.begin(), nums.end()); //for using two pointers technique
+            
+        for(int i=0; i<n-2; i++){
+            //now using two pointers to find the second and third
+            
+            if(i >0 && nums[i] == nums[i-1])
+                continue;
+            int start = i+1, end= n-1;
+            while(start < end){
+                int sum = nums[i] + nums[start]+ nums[end];
+                if(sum == 0)
+                { ans.insert({nums[i],nums[start], nums[end]}); start +=1;}
+                
+                else if(sum > 0)
+                    end -=1;
+                else
+                    start +=1;
+               
+            }
+        }
+        
+        vector<vector<int>> yo;
+        for(auto i= ans.begin(); i != ans.end(); i++)
+            yo.push_back(*i);
+        return yo;
+    }
+};