Given a non-negative integer
num, repeatedly add all its digits until the result has only one digit.
num = 38, the process is like:
3 + 8 = 11,
1 + 1 = 2. Since
2 has only one digit, return it.
Could you do it without any loop/recursion in O(1) runtime?
Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.