(1)Two Sum - O(n)

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        unordered_map<int,int>mp;
        int n = nums.size();
        
        for(int i=0 ; i<n ; i++){
            if(mp.find(target-nums[i]!=mp.end()))
                return {mp[target.nums[i]]};
            mp[nums[i]=i];
        }
        return {};
    }
};