Leetcode.1 两数之和

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