leetcode-258-easy

Add Digits

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?

思路一：模拟，直接循环求解

public int addDigits(int num) {
    int temp = num;
    while (num > 9) {
      temp = 0;
      while (num > 0) {
        temp += num % 10;
        num /= 10;
      }
        num = temp;
    }

    return temp;
}

思路二：数学，(num - 1) % 9 + 1