Pairs (two pointer approach).java

import java.util.Scanner;
import java.util.Arrays;

public class Playground {

	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);

		int n = scanner.nextInt();
		int k = scanner.nextInt();
		
		int a[] = new int[n];
		for (int i = 0; i < n; i++) {
			a[i] = scanner.nextInt();
		}
		Arrays.sort(a);
		
		if (n == 0 || n == 1) {
			System.out.println(0);
			return;
		}
		
		int aIndex = 1;
		int bIndex = 0;
		
		int count = 0;
		while(aIndex + 1 != n || bIndex + 1 != n) {
			int abs = Math.abs(a[aIndex] - a[bIndex]);
			if (abs == k) {
				count++;
				if (aIndex + 1 == n) {
					bIndex++;
				} else {
					aIndex++;
				}
			} else if (abs > k) {
				bIndex += bIndex + 1 == n ? 0 : 1;
			} else {
				if (aIndex + 1 == n) {
					bIndex++;
				} else {
					aIndex++;
				}
			}
		}
		System.out.println(count);
	}
}