leetcode 1. Sum of two numbers

Given an array of integers and a target value, find two numbers in the array that sum to the target value.

You can assume that each input corresponds to only one answer, and that the same elements cannot be reused.

Example:

Given nums = [2, 7, 11, 15], target = 9

because nums[0] + nums[1] = 2 + 7 = 9
So return [0, 1]

 1 class Solution {
 2 public:
 3     vector<int> twoSum(vector<int>& nums, int target) {
 4         vector<int> ans;
 5         bool flag = false;
 6         for(int i = 0; i < nums.size()-1; i++){
 7             for (int j = i+1; j < nums.size(); j++) {
 8                 if(nums[i]+nums[j] == target) {
 9                     ans.push_back(i);
10                     ans.push_back(j);
11                     flag = true;
12                     break;
13                 }
14             }
15             if(flag) break;
16         }
17         return ans;
18     }
19 };