A1069 The Black Hole of Numbers (14/20 分)

#include <iostream>
#include <algorithm>
#include <sstream>
using namespace std; 

bool cmp(char c1, char c2){
  return c1 > c2;
}

//string -> int
int change(string s){
  int n;
  stringstream ss;
  ss << s;
  ss >> n;
  return n;
}
//int -> string
string change2(int n){
  string s;
  stringstream ss;
  ss << n;
  ss >> s;
  return s;
}
int main(int argc, char** argv) {
  string s1, s2;
  cin >> s1;
  
  while(1){
    s2 = s1;
    sort(s1.begin(), s1.end());
    sort(s2.begin(), s2.end(), cmp);
    
    int num1 = change(s1);
    int num2 = change(s2);
    int num = num2- num1;
    printf("%04d - %04d = %04d\n", num2, num1, num);
    if(num == 6174){
      break;
    } else if(num == 0){
      break;
    } else {
      s1 = change2(num);
    }
  }
  return 0;
}