adding two sum leetcode problem

leetcode/p1_twosum.cpp

@@ -0,0 +1,25 @@
+class Solution {
+public:
+    vector<int> twoSum(vector<int>& nums, int target) {
+       vector<int> ans;
+       int size = nums.size();
+       // 1. create the map
+       // key: number; value: index;
+       map<int,int> map1;
+       for(int i=0;i<size;i++){
+           map1[nums[i]] = i;
+       }
+       // 2. use the map
+       // nums[i]==3 and target=6
+       for(int i=0;i<size;i++){
+           // if target-nums[i] also exists.
+           if(map1.find(target-nums[i])!=map1.end() && i!=map1[target-nums[i]]){
+                ans.push_back(i);
+                ans.push_back(map1[target-nums[i]]);
+                break;
+           }
+       }
+
+       return ans;
+    }
+};