vector<int> twoSum(vector<int>& nums, int target) {

int n = nums.size();

unordered_map<int, int> mp; // val -> index

for (int i = 0; i < n; i++) {

int complement = target - nums[i];

if (mp.find(complement) != mp.end()) {

return {mp[complement], i};

}

mp.insert({nums[i], i});

}

return {};

}