1. Two Sum

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