PAT 1069. The Black Hole of Numbers

#include<iostream>
#include<cstdio>
#include<cstring> 
#include<algorithm>
using namespace std;
bool cmp1(int a,int b)
{
    if(a>b) return true;
	return false;
}
bool cmp2(int a,int b)
{
    if(a<b) return true;
	return false;
}
int main()
{
	int i,n,a[4];
	scanf("%d",&n);
	a[3]=n%10;
	a[2]=(n/10)%10;
	a[1]=(n/100)%10;
	a[0]=n/1000;
	if((a[0]==a[1])&&(a[1]==a[2])&&(a[2]==a[3])) printf("%04d - %04d = 0000\n",n,n);
	else 
	{
	   while(true)
	   {
	      sort(a,a+4,cmp1);
	      int num1=a[0]*1000+a[1]*100+a[2]*10+a[3];	   
	      sort(a,a+4,cmp2);
	      int num2=a[0]*1000+a[1]*100+a[2]*10+a[3];
          printf("%04d - %04d = %04d\n",num1,num2,num1-num2);
		  if(num1-num2==6174) break;
		  num1=num1-num2;
		  a[3]=num1%10;
	      a[2]=(num1/10)%10;
	      a[1]=(num1/100)%10;
	      a[0]=num1/1000;
	   }
	}
	return 0;
}