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;
}