1 Given nums = [2, 7, 11, 15], target = 9,
2
3 Because nums[0] + nums[1] = 2 + 7 = 9,
4 return [0, 1].

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

class Solution {
public:vector<int> twoSum(vector<int>& nums, int target){unordered_map<int, int> m;vector<int> vi;for (int i=0;i<nums.size();i++){m[nums[i]] = i;}for (int i=0;i<nums.size();i++){int other_number = target - nums[i];if (m.count(other_number) && m[other_number]!=i){vi.push_back(i);vi.push_back(m[other_number]);break;}}return vi;}
};

class Solution {
public:vector<int> twoSum(vector<int>& nums, int target){unordered_map<int, int> numer_to_id;for (int i=0;i<nums.size();i++){if (numer_to_id.count(target-nums[i])){return{ i,numer_to_id[target - nums[i]] };}numer_to_id[nums[i]] = i;}return{};}
};

 1 class Solution(object):
 2     def twoSum(self, nums, target):
 3         """
 4         :type nums: List[int]
 5         :type target: int
 6         :rtype: List[int]
 7         """
 8         size = len(nums)
 9         i = 0
10         for i in range(size):
11             for j in range(i+1,size):
12                 if nums[i]+nums[j] == target:
13                     return [i,j]