258. Add Digits

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.
Example:
Input: 38
Output: 2
Explanation: The process is like: 3 + 8 = 11, 1 + 1 = 2.
Since 2 has only one digit, return it.

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题解：

#include <iostream>
#include <string>

using std::cout;
using std::endl;
using std::to_string;

int AddDigits(int num);

int Sum(int num);

int main(int argc, const char* argv[]) {
    int num = 38;
    cout << AddDigits(num) << endl;

    system("pause");
    return 0;
}

int AddDigits(int num) {
    do {
        num = Sum(num);
    } while (to_string(num).size() != 1);
    return num;
}

int Sum(int num) {
    if (num == 0)
        return 0;

    return num % 10 + Sum(num / 10);
}