leetcode 258: add digits

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

代码

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int (int num) {
    int res = num % 9;
    return (res != 0 || num == 0) ? res : 9;
}