[PAT] 1069 The Black Hole of Numbers （20 分）Java

For any 4-digit integer except the ones with all the digits being the same, if we sort the digits in non-increasing order first, and then in non-decreasing order, a new number can be obtained by taking the second number from the first one. Repeat in this manner we will soon end up at the number 6174 -- the black hole of 4-digit numbers. This number is named Kaprekar Constant.

For example, start from 6767, we'll get:

7766 - 6677 = 1089
9810 - 0189 = 9621
9621 - 1269 = 8352
8532 - 2358 = 6174
7641 - 1467 = 6174
... ...

Given any 4-digit number, you are supposed to illustrate the way it gets into the black hole.

Input Specification:

Each input file contains one test case which gives a positive integer N in the range (0,10​4​​).

Output Specification:

If all the 4 digits of N are the same, print in one line the equation N - N = 0000. Else print each step of calculation in a line until 6174 comes out as the difference. All the numbers must be printed as 4-digit numbers.

Sample Input 1:

6767

Sample Output 1:

7766 - 6677 = 1089
9810 - 0189 = 9621
9621 - 1269 = 8352
8532 - 2358 = 6174

Sample Input 2:

2222

Sample Output 2:

2222 - 2222 = 0000

package pattest;

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

/**
 * @Auther: Xingzheng Wang
 * @Date: 2019/2/26 10:17
 * @Description: pattest
 * @Version: 1.0
 */
public class PAT1069 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int num = sc.nextInt();
        do {
            num = blackHole(num);
        } while (num != 6174 && num != 0);
    }

    private static int blackHole(int n) {
        String s = Integer.toString(n);
        char[] c = s.toCharArray();
        Arrays.sort(c);
        int[] num = new int[4];
        for (int i = 0; i < c.length; i++) {
            num[3 - i] = c[c.length - i - 1] - '0';
        }

        int big, small, temp;
        small = num[0] * 1000 + num[1] * 100 + num[2] * 10 + num[3];
        big = num[3] * 1000 + num[2] * 100 + num[1] * 10 + num[0];
        temp = big - small;
        if (big == temp) {
            System.out.printf("%04d - %04d = %04d", big, small, temp);
        } else {
            System.out.printf("%04d - %04d = %04d", big, small, temp);
        }
        if (temp != 6174 && temp != 0) {
            System.out.println();
        }
        return temp;
    }
}