[LeetCode][JavaScript]Add Digits

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.

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

Hint:

1. A naive implementation of the above process is trivial. Could you come up with other methods?
2. What are all the possible results?
3. How do they occur, periodically or randomly?
4. You may find this Wikipedia article useful.

如果可以用循环，最好的情况是读一遍数字，O(1)时间复杂度。

贪心，每读一位数都做求Digit Root的操作，最后的结果是正确的。

 1 /**
 2  * @param {number} num
 3  * @return {number}
 4  */
 5 var addDigits = function(num) {
 6     var str = num.toString(), res = 0, tmp1, tmp2;
 7     for(var i = 0; i < str.length; i++){
 8         res = parseInt(str[i]) + res;
 9         if(res >= 10){
10             tmp1 = parseInt(res / 10);
11             tmp2 = res % 10;
12             res = tmp1 + tmp2;
13         }
14     }
15     return res;
16 };

最佳答案参照wiki ： https://en.wikipedia.org/wiki/Digital_root

好几种写法都可以，这里列了2种。

1 /**
2  * @param {number} num
3  * @return {number}
4  */
5 var addDigits = function(num) {
6     return num === 0 ? 0 : num - 9 * Math.floor((num - 1) / 9);
7 };

1 /**
2  * @param {number} num
3  * @return {number}
4  */
5 var addDigits = function(num) {
6     return 1 + (num - 1) % 9;
7 };