Count Pairs Divisible by K

You are given an array ARR and a positive integer K. Your task is to count the total number of pairs within the array whose sum is divisible by K.

Example:

Input:

ARR = [4, 3, 5, 1, 4, 5], K = 5

Output:

3

Explanation:

The pairs whose sums are divisible by 5 are [4, 1], [5, 5], and [4, 1]. Thus, there are 3 pairs.

Constraints:

• 1 <= T <= 10
• 2 <= N < 10^6
• 1 <= K <= 10^9
• 0 <= arr[i] <= 10^9
• Time Limit: 1 sec

Note:

You do not need to print anything. It has already been taken care of. Just implement the function.