Solution.cs

/*
    Problem: https://www.hackerrank.com/challenges/divisible-sum-pairs/problem
    C# Language Version: 6.0
    .Net Framework Version: 4.7
    Tool Version : Visual Studio Community 2017
    Thoughts :
    1. Let all the input integers are stored in an array arr of length n.
    2. Let the divisor be k.
    3. Let the total divisible pair count be d. Initialize d to 0.
    4. Start a loop which runs with a counter i starting from 0 to n-1.
    4.1 Start a nested loop which runs with a counter j starting from i+1 to n-1
        4.1.1 If arr[i] + arr[j] is wholely divisible by k then increment d by 1.
        4.1.2 Repeat step 4.1.1 for all the array elements until loop termination condition is met.
    4.2 Repeat step 4.1 for all the array elements until loop termination condition is met.
    5. Print d.

    Time Complexity:  O(n^2) //there is a nested loop.
    Space Complexity: O(1) //number of dynamically allocated variables remain constant for any input.
                        The array allocation is part of the input being fed into algorithm steps.
                
*/

using System;
using static System.Console;

class Solution
{
    static int DivisibleSumPairs(int k, int[] ar)
    {
        var divisiblePairCount = 0;
        for (int i = 0; i < ar.Length; i++)
        {
            for (int j = i + 1; j < ar.Length; j++)
            {
                if ((ar[i] + ar[j]) % k == 0)
                    divisiblePairCount++;
            }
        }

        return divisiblePairCount;
    }

    static void Main(String[] args)
    {
        var tokens_n = ReadLine().Split(' ');
        var k = int.Parse(tokens_n[1]);
        var ar_temp = ReadLine().Split(' ');
        var ar = Array.ConvertAll(ar_temp, int.Parse);
        var result = DivisibleSumPairs(k, ar);
        WriteLine(result);
    }
}