PAT 1069
技巧题
#include <cstdio>
#include <iostream>
#include <cmath>
#include <algorithm>
#include <string>
using namespace std;
bool cmp(char a,char b)
{
return a>b;
}
int main()
{
string s;
cin>>s;
s.insert(0,4-s.length(),'0');
do{
string a=s,b=s;
sort(a.begin(),a.end(),cmp);
sort(b.begin(),b.end());
int result=stoi(a)-stoi(b);
s=to_string(result);
s.insert(0,4-s.length(),'0');
cout<<a<<" - "<<b<<" = "<<s<<endl;
}while( s!="6174" && s!="0000" );
return 0;
}