lleetcode 1 two sum c++

 1 class Solution {
 2 public:
 3     vector<int> twoSum(vector<int>& nums,int target) {
 4         vector<int> res;
 5         for(int i = 0;i < nums.size();i++)
 6             for(int j = i + 1;j < nums.size();j++)
 7                 if(nums[i]+nums[j]==target)
 8                 {
 9                     res.push_BACk(i);
10                     res.push_BACk(j);
11                 }
12         return res;
13                     
14     }
15 };

 1 class Solution {
 2 public:
 3     vector<int> twoSum(vector<int>& nums,int target) {
 4         vector<int> res;
 5         unordered_map<int,int> map;
 6         for(int i = 0; i < nums.size();i++)
 7         {
 8             map[nums[i]] = i;
 9         }
10         for(int i = 0;i < nums.size();i++)
11         {
12             int left = target - nums[i];
13             if(map.count(left) && i <  map[left])
14             {
15                 res.push_BACk(i);
16                 res.push_BACk(map[left]);
17             }        
18         } 
19         return res;
20     }
21 };