PAT 1069. The Black Hole of Numbers

题目：http://pat.zju.edu.cn/contests/pat-a-practise/1069

题解：

代码：

#include<cstdio>
#include<iostream>
#include<cstring>
#include<cmath>
#include<string>
#include<vector>
#include<algorithm>
using namespace std;
char a[5],b[5],c[5];
int main()
{
    int ans=0;
    int temp;
    int x,y;
    scanf("%d",&temp);
    a[3]=temp%10+'0';
    a[2]=(temp/10)%10+'0';
    a[1]=(temp/100)%10+'0';
    a[0]=(temp/1000)+'0';
    if(a[0]==a[1]&&a[1]==a[2]&&a[2]==a[3])
    {
        printf("%s - %s = 0000\n",a,a);
        return 0;
    }
    for(;;)
    {
        strcpy(b,a);
        sort(a,a+4,greater<char>());
        sort(b,b+4);
        ans=0;
        for(int i=3; i>=0; --i)
        {
            x=a[i]-'0';
            y=b[i]-'0';
            if(x<y+ans)
            {
                c[i]=x+10-y-ans+'0';
                ans=1;
            }
            else
            {
                c[i]=x-y-ans+'0';
                ans=0;
            }
        }
        printf("%s - %s = %s\n",a,b,c);
        strcpy(a,c);
        if(strcmp(a,"6174")==0)
            break;
    }
    return 0;
}

来源： http://blog.youkuaiyun.com/acm_ted/article/details/20147625