PAT1019 数字黑洞

又被耍了，输入不保证四位数字

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

void o(string &st){
   sort(st.begin(),st.end());
   string ss=st;
   reverse(ss.begin(),ss.end());
   int k=atoi(ss.c_str())-atoi(st.c_str());

   string result(4,'0');
   for(int i=3;i>=0;--i){result[i]+=k%10;k/=10;}
   cout<<ss<<" - "<<st<<" = "<<result<<endl;
   st=result;
}

void out(string &st){
st=string(4-st.size(),'0')+st;
sort(st.begin(),st.end());
if(st[0]==st[3]){cout<<st<<" - "<<st<<" = 0000\n";return;}
while(st!="6174")o(st);
}

int main(){
string st;
cin>>st;
out(st);
return 0;}