1069. The Black Hole of Numbers (20) string转int

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

bool compare(char a, char b){ return a > b;}

int main(int argc, const char * argv[]) {
    string s, a, b;
    cin >> s;
    s.insert(0, 4 - s.length(), '0');
    
    do{
        a = s;
        b = s;
        sort(a.begin(), a.end());
        sort(b.begin(), b.end(), compare);
        int result = stoi(b) - stoi(a);
        s = to_string(result);
        s.insert(0, 4 - s.length(), '0');
        cout << b << " - " << a << " = " << s << endl;
    }while (s != "6174" && s != "0000");
    
    return 0;
}