https://www.hackerrank.com/challenges/pairs/problem?h_l=interview&playlist_slugs%5B%5D=interview-preparation-kit&playlist_slugs%5B%5D=search

Pairs.java

import java.io.*;
import java.math.*;
import java.security.*;
import java.text.*;
import java.util.*;
import java.util.concurrent.*;
import java.util.regex.*;

public class Solution {

    // Complete the pairs function below.
    static int pairs(int k, int[] arr) {
        // 22:23 THA 24/02/2019
        // by BingLi224
        // Reference: https://www.hackerrank.com/challenges/pairs/problem?h_l=interview&playlist_slugs%5B%5D=interview-preparation-kit&playlist_slugs%5B%5D=search
    
        // sort the array first
        Arrays.parallelSort ( arr );

        int l = 0;
        int r = 1;
        int sz = arr.length;
        int maxL = sz - 1;
        int pair = 0;
        int total = 0;
        while ( l < maxL ) {
            pair = arr [ l ] + k;
            
            // find pair
            while ( r < sz && pair > arr [ r ] ) {
                r ++;
            }

            if ( r >= sz )
                break;

            if ( arr [ r ] == pair ) {
                // found pair
                total ++;
                r ++;
            }

            // find next
            l ++;
        }

        return total;
    }

    private static final Scanner scanner = new Scanner(System.in);

    public static void main(String[] args) throws IOException {
        BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(System.getenv("OUTPUT_PATH")));

        String[] nk = scanner.nextLine().split(" ");

        int n = Integer.parseInt(nk[0]);

        int k = Integer.parseInt(nk[1]);

        int[] arr = new int[n];

        String[] arrItems = scanner.nextLine().split(" ");
        scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");

        for (int i = 0; i < n; i++) {
            int arrItem = Integer.parseInt(arrItems[i]);
            arr[i] = arrItem;
        }

        int result = pairs(k, arr);

        bufferedWriter.write(String.valueOf(result));
        bufferedWriter.newLine();

        bufferedWriter.close();

        scanner.close();
    }
}