[LeetCode] 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:

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.

解題思路

思路1：一直相加直到num變為個位數。
思路2：套用維基公式。

實現代碼

Java代碼 1：

// Runtime: 2 ms
public class Solution {
    public int addDigits(int num) {
        int temp = 0;
        while (num > 9) {
            while (num != 0) {
                temp += num % 10;
                num /= 10;
            }
            num = temp;
            temp = 0;
        }

        return num;
    }
}

Java代碼2：

// Runtime: 5 ms
public class Solution {
    public int addDigits(int num) {
        return (int)(num - 9 * Math.ceil((num - 1) / 9));
    }
}

Java代碼3：

// Runtime: 2 ms
public class Solution {
    public int addDigits(int num) {
        return 1 + (num - 1) % 9;
    }
}