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.

Follow up:
Could you do it without any loop/recursion in O(1) runtime?

int addDigits(int num) 
{
    if(num==0)
        return 0;
    return num%9==0?9:num%9;
}

经验总结

这类题我认为在LeetCode上是最没意思的题目，因为实质上是数学问题（归纳问题）。
如下规律：
input: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16……
output: 0 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 …