PAT 1069. The Black Hole of Numbers

#include <cstdio>
#include <cstdlib>
#include <vector>
#include <algorithm>

using namespace std;

int ensure(int n) {
    if (n < 10) {
       n *= 1000;
    } else if (n < 100) {
       n *= 100;       
    } else if (n < 1000) {
       n *= 10;       
    }
    return n;
}

void extend_print(int n) {
     if (n < 10) {
        printf("000%d", n);      
     } else if (n < 100) {
        printf("00%d", n);       
     } else if (n < 1000) {
        printf("0%d", n);
     } else {
        printf("%d", n);
     }
}

void print_equal(int a, int b, int c) {
     extend_print(a);
     printf(" - ");
     extend_print(b);
     printf(" = ");
     extend_print(c);
     printf("\n");    
}

bool mycmp(int a, int b) {
     if (a >= b) return true;
     return false;     
}

int reverse(int n) {
    int v = 0;
    while (n) {
          v = v * 10 + n % 10;
          n/= 10;
    }
    return v; 
}

int sort_digits(int n) {
    int ds[5] = {0};
    int len = 0;
    while (n) {
          ds[len++] = n % 10;
          n/=10;
    }
    if (len == 0) return 0;
    sort(ds, ds + len, mycmp);
    int v = 0;
    for (int i=0; i<len; i++) {
        v = v * 10 + ds[i];    
    }
    return ensure(v);
}

int main() {
    int n;
    scanf("%d", &n);
    
    int desc = sort_digits(n);
    int asc = reverse(desc);
    int res = desc - asc;
    print_equal(desc, asc, res);
    
    while (res != 6174 && res != 0) {
        desc = sort_digits(res);
        asc = reverse(desc);
        res = desc - asc;
        print_equal(desc, asc, res);
    }
    system("pause");
    return 0;    
}