LeetCode-1 | Two sum | C++

Two sum - Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

TIP - always think of brute force algorithm first and come up with enhanced code later.

Brute force solution :

It’s done using two nested for loops .

Time complexity — O(n²)

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        int i,j;
        vector<int> a;
        for(i=0;i<nums.size();i++){
            for(j=i+1;j<nums.size();j++){
                if(nums[i]+nums[j] == target){
                    a.push_back(i);
                    a.push_back(j);
                }    
            }
        }
        
        return a;
    }
};

O(n) solution :

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