1069. The Black Hole of Numbers

没有说一定是4为数字

#include<string>
#include<iostream>
#include<algorithm>
using namespace std;
string step(string st)
{
    sort(st.begin(),st.end());
    string ss(st.size(),0);
    using RI=reverse_iterator<string::iterator>;
    copy(st.begin(),st.end(),RI(ss.end()));
    string re=to_string(stoi(ss)-stoi(st));
    re=string(4-re.size(),'0')+re;
    cout<<ss<<" - "<<st<<" = "<<re<<endl;
    return re;
}
int main()
{
    string st;
    cin>>st;
    st=string(4-st.size(),'0')+st;
    if(all_of( st.begin(), st.end(), [&] (char t) {return t==st[0]; }  ))
        cout<<st<<" - "<<st<<" = 0000\n";
    else while((st=step(st))!="6174");
    return 0;
}