【编程题】数字黑洞

　　附上Java实现代码。

package NetEasy;

/**
 * 数字黑洞
 */

import java.util.Scanner;

public class NumBlackHole {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String number = in.next();
//        StringBuffer numStr = new StringBuffer();
//        numStr.append(number);
        if(number.charAt(0)==number.charAt(1) &&
                number.charAt(0)==number.charAt(2)&&
                number.charAt(0)==number.charAt(3))
            System.out.println(number+" - "+number+" = 0");
        else{
            boolean flag = true;
//            int num = Integer.valueOf(number);
            while(flag){
                number = computeNum(number);
                if(number.equals("6174"))
                    flag = false;
            }
        }

    }

    private static String computeNum(String number) {
//        String number = Integer.valueOf(num).toString();
        StringBuffer numOrder = new StringBuffer(number);
        for(int i = 0; i < 4; i++){
            for(int j = i; j < 4; j++){
                if(numOrder.charAt(i) < numOrder.charAt(j)){
                    char temp = numOrder.charAt(i);
                    numOrder.setCharAt(i, number.charAt(j));
                    numOrder.setCharAt(j, temp);
                }
            }
        }
//        StringBuffer numUpper = numOrder;  //这样复制，后面numOrder反转了 numUpper会跟着变化
        String numUpper = numOrder.toString();
        String numLower = numOrder.reverse().toString();
        int n = Integer.valueOf(numUpper)-Integer.valueOf(numLower);
        StringBuffer result = new StringBuffer(Integer.valueOf(n).toString());
        if(result.length() == 1)
            result.insert(0, "000");
        if(result.length() == 2)
            result.insert(0, "00");
        if(result.length() == 3)
            result.insert(0, "0");
        String resultnum = result.toString();
        System.out.println(numUpper+" - "+numLower+" = "+ resultnum);
        return resultnum;
    }

}