题解 | #Digital Roots#

#include<iostream>
#include<string>

using namespace std;

int char2int(char c) {
    return c - '0';
}
char int2char(int d) {
    return d + '0';
}

int main() {
    string number_str;
    while (cin >> number_str) {
        while (number_str.size() != 1) {
            int sum = 0;
            for (int i = 0; i < number_str.size(); i++) {
                sum = sum + char2int(number_str[i]);
            }
            number_str = to_string(sum);
        }
        cout << number_str << endl;
    }

}