213. Two Sum
# @Time complexity: O(n)
class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        unordered_map<int,int> mp;
        for(auto i=0;i<nums.size();i++){
            if(mp.find(target-nums[i])!=mp.end()){
                return {mp[target-nums[i]],i};
            }
            else{
                mp[nums[i]]=i;
            }
        }
        return {};
    }
};