一、题目

Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.

Example 1:

Input: num = 38
Output: 2
Explanation: The process is
38 --> 3 + 8 --> 11
11 --> 1 + 1 --> 2
Since 2 has only one digit, return it.
Example 2:

Input: num = 0
Output: 0

Constraints:

0 <= num <= 231 - 1

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

二、题解

class Solution {
public:int addDigits(int num) {while(num > 9){int sum = 0;while(num){sum += num % 10;num /= 10;}num = sum;}return num;}
};