使用哈希表实现 时间复杂度 O(N)
vector<int> twoSum(vector<int>& nums, int target) {
unordered_map<int, int> hashtable;
for (int i = 0; i < nums.size(); i++) {
auto item = hashtable.find(target - nums[i]);
if (item != hashtable.end()) return {item->second, i};
hashtable[nums[i]] = i;
}
return {};
}