Interview Question Preperation : Find longest subarray whose sum is equal to K

Software Engineering Practice interview question

Given a array of N elements. Find the length of Longest Subarray whose sum is equal to given K.

 This I believe is one of the most basic Problem Solving(PS) question, which I have found most people get wrong. 

So I wont keep you holding for long, and here is the solution: 

//Returns -1 when no sub-array has sum equal to k. 

int longestSubarrayWithSumK(vector<int> inputArray, long k)     

int l = 0,  max_len = -1, n = inputArray.size();    

    for(int r = 0; r < n; r++){

        long long running_sum= running_sum+ inputArray[r];

        //when the running_sum overshoots the k, try to get it under K and note what the index untill where you are reducing it. 

        while(running_sum> k){

            running_sum -= inputArray[l];

            l++;

        }

     //Once we get the running sum equal to K, check if the current running index length of subarray is more than previously seen subarray length

        if (running_sum== k){

            max_len = max(max_len, r - l + 1);

        }    

    }

        return max_len;

}