Add Digits

Total Accepted: 73983 Total Submissions: 153758 Difficulty: Easy
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:

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

Wikipedia

思路

public class Solution {
    public int[] plusOne(int[] digits) {

        int n = digits.length;
        for(int i=n-1; i>=0; i--) {
            if(digits[i] < 9) {
                digits[i]++;
                return digits;
            }

            digits[i] = 0;
        }

        int[] newNumber = new int [n+1];
        newNumber[0] = 1;

        return newNumber;
    }
}