1069. The Black Hole of Numbers

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, 10000).

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.

这题跟B19是一样的

#include <stdio.h>
#include <stdlib.h>
#include <algorithm>
using namespace std;
int Sort1(int a1,int a2,int a3,int a4);
int Sort2(int a1,int a2,int a3,int a4);
int main ()
{
    int a1=0,a2=0,a3=0,a4=0,i=0;
    int num;
    scanf("%d",&num);
    a1=num/1000;
        a2=(num/100)%10;
        a3=(num%100)/10;
        a4=num%10;

    if(a1==a2 && a2==a3 && a3==a4)
    {
        printf("%d%d%d%d - %d%d%d%d = 0000\n",a1,a1,a1,a1,a1,a1,a1,a1);
        return 0;
        }
    int result=0,first=0,second=0;
    while(1)
    {
        first=Sort2(a1,a2,a3,a4);
        second=Sort1(a1,a2,a3,a4);
        result=first-second;
        a1=result/1000;
        a2=(result/100)%10;
        a3=(result%100)/10;
        a4=result%10;
        printf("%04d - %04d = %04d\n",first,second,result);
        if(result==6174||result==0) break;
        }

    system("pause");
    return 0;
    }

int Sort1(int a1,int a2,int a3,int a4)
{
     int a[4]={a1,a2,a3,a4};
     sort(a,a+4);
     return (a[0]*1000+a[1]*100+a[2]*10+a[3]);
     }
bool cmp(int a,int b)
{
     return a>b;
     }
int Sort2(int a1,int a2,int a3,int a4)
{
     int a[4]={a1,a2,a3,a4};
     sort(a,a+4,cmp);
     return (a[0]*1000+a[1]*100+a[2]*10+a[3]);
     }