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.

找规律

class Solution {public:    int addDigits(int num) {        return 1 + (num - 1) % 9;     }};

leetcode 258. Add Digits