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.

这个题目就是说不断的把整数拆分成个位数再相加，直到不能相加为止，要求复杂度是O(1)。 那么可以采用递归算法再来做。

/** * @param {number} num * @return {number} */var addDigits = function(num) {    if(num<10)return num;    else return addDigits(addDigits(Math.floor(num/10))+num%10);};