边插入hash表，边寻找。

class Solution { 
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        vector<int> res;
        unordered_map<int, int> mp;
        for(int i=0;i<nums.size();i++){
            int another = target - nums[i];
            if(mp.count(another)){
                 res = vector<int>({mp[another], i});
                return res;
            }
            mp[nums[i]] = i;
        }
        return res; //没找到要返回空
    }
};