leetcode two sum

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