1. Two Sum

https://leetcode.com/problems/two-sum/description/

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