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