Leetcode 258. Add Digits

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

<span style="font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif; background-color: rgb(255, 255, 255);"></span>

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?

public class Solution {
    public int addDigits(int num) {
        if (num == 0){
            return 0;
        }
        if (num % 9 == 0){
        return 9;
        }
        else{
            return (num % 9);
        }
    }
}

把两位数依次列出，列十几个就能找到规律

discuss最短代码，思路一样

public int addDigits(int num) {
    return num == 0 ? 0 : (num % 9 == 0 ? 9 : num % 9);
}