class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        // 将所有数都放入hash表
        // 扫描到第Si个数时, 查看target-Si是否在hash表中 O(n)
        unordered_map<int, int> heap;
        for(int i = 0; i < nums.size(); i++){
            int query = target - nums[i];
            if(heap.count(query)) return {heap[query], i};
            heap[nums[i]] = i;
        }
        return {};
    }
};