Minimum Sum of Four Digit Number After Splitting Digits

<-E 2160> Minimum Sum of Four Digit Number After Splitting Digits

class Solution {
public:
    int minimumSum(int num) {
        string s = to_string(num);
        sort(s.begin(), s.end());

        return (s[0] - '0' + s[1] - '0') * 10
        + (s[2] - '0') + (s[3] - '0');
    }
};