算法系列（9） LeetCode256

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?

/** * @param {number} num * @return {number} */var addDigits = function(num) {    while(num.toString().length>1){        var sum=0;        while(num!=0){            sum += num%10;            num = ~~(num/10);        }            num = sum;    }    return num;};