PAT (Basic Level) 1007 for prime conjecture (20 points) JAVA solution

Sample input:

20

Sample output:

4

import java.util.ArrayList;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner (System.in);
		ArrayList<Integer> al = new ArrayList<>();
		int N = sc.nextInt();
		for (int i = 0; i < N+1; i++) {
			if(isPrime(i)) {
				al.add(i);
			}
		}
		int count=0;
		for (int i = 0; i < al.size()-1; i++) {
			if(al.get(i+1).hashCode()-al.get(i).hashCode()==2) {
				count++;
			}
		}
		System.out.println(count);

	}

	
	
	public static boolean isPrime(int num) {
		for (int i = 2; i * i<=num; i++) {
			if(num%i==0)return false;
		}
		return true;
	}
}