PAT（乙级）1019

#include<stdio.h>
#include<stdlib.h>

int comp(const void*a, const void*b)
{
  return *(int*)b - *(int*)a;
}

int main()
{
  int num[4];
  int max, min, n;
  scanf("%d",&n);
    //分离数字
    num[0] = n / 1000;
    num[1] = n / 100 % 10;
    num[2] = n / 10 % 10;
    num[3] = n % 10;

  int temp;
  while (1)
  {
    qsort(num, 4, sizeof(int), comp);		//排序
    max = 1000 * num[0] + 100 * num[1] + 10 * num[2] + num[3];
    min = 1000 * num[3] + 100 * num[2] + 10 * num[1] + num[0];

    temp = max - min;
    if (max == min)
    {
      printf("%04d - %04d = 0000", max, max);
      return 0;
    }
    else
    {
      printf("%04d - %04d = %04d\n", max, min, temp);
      if (6174 == temp)
      {
        return 0;
      }

      num[0] = (temp / 1000);
      num[1] = temp / 100 % 10;
      num[2] = temp / 10 % 10;
      num[3] = (temp % 10);
    }

  }
  return 0;
}