1069. The Black Hole of Numbers

#include<cstdio>#include<cstdlib>#include<algorithm>using namespace std;bool cmp(char a,char b){    return a > b;}int main(){    int n,a,b,c;    char s[4];    scanf("%d",&n);    do{        for(int i = 3;i >= 0;i --){            s[i] = n%10 + '0';            n /= 10;        }        sort(s,s + 4);        b = atoi(s);        sort(s,s + 4,cmp);        a = atoi(s);        c = a - b;        printf("%04d - %04d = %04d\n",a,b,c);        n = c;    }while(c != 6174 && c != 0);    return 0;}