Hi Devs, Happy Coding
I am planning to revise Data Structure and Algorithms Concepts so I will be sharing my approach as well as a full solution for problems I solve. This is my first attempt at writing a dev blog, yeah I am a bit nervous. Hope you like it and Your valuable feedback and contributions are always welcome.

Today I am going to solve a coding problem Diffk which is based on the Two Pointer concept.

Asked In - Facebook Interview

Problem Statement - Given an array ‘A’ of sorted integers and another non negative integer k, find if there exists 2 indices i and j such that A[i] - A[j] = k, i != j.

Time and Space Complexity Constraint

1. Try doing this in less than linear space complexity.

Example

Input :
  A : [1 3 5] 
  k : 4
Output : YES
Explanation : as 5 - 1 = 4

Brute Force Approach - we use two loop for two indices i,j and iterate if we find any A[j]-A[i] = k and i!=j then return 1 else 0.

int diffPossible(vector<int> &A, int k) {
  int N= A.size();
  for (int i = 0; i < N; i++) {
    for (int j = i + 1; j < N; j++) {
      if (A[j] - A[i] > k) break; // No need check forward because Array is sorted so the difference is going to increase even further because A[I] is going to increase or remain same.
      if (A[j] - A[i] == k) return 1; 
    }
  }
  return 0;
}

The Time Complexity of this approach is O(N*N).

Observation
Let's check the above approach when we have i = I and find j=J ,Diff_1 =(A[J-1]-A[I]) such that Diff_1 <k and A[J]-A[I] >k

• This means that for i =I and j> J we are not going to find our solution because A[j] will be increasing(Array is sorted) so will A[j]-A[I].

• when i increase then A[i] will increase too then A[J]- A[i] will decrease so till j =J-1 our A[j] -A[i] will be less than Diff_1 which is also less than k( Observation 1 ) So we will not find our solution till j =J-1. So Efficient Approach will be when we don’t start j every time with I+1 but start with j=J.

New Approach Code (Time Complexity: O(n))

int diffPossible(vector<int> &A, int k) {
    int N = A.size();
    int j = 0; 
    for (int i = 0; i < N; i++) {
        j = max(j, i+1);
        while (j < N && (A[j] - A[i] < B)) {
            j += 1;
        }
        if (A[j] - A[i] == B) return 1;
    }
    return 0;
}

Github Solution Link