258. Add Digits

258. Add Digits (Easy)

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

Example:

Input: 38
Output: 2 
Explanation: 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?

Related Topics:
Math

Similar Questions:

• Happy Number (Easy)
• Sum of Digits in the Minimum Number (Easy)

Solution 1.

See explanation in https://leetcode.com/problems/add-digits/solution/.

// OJ: https://leetcode.com/problems/add-digits/
// Author: github.com/lzl124631x
// Time: O(1)
// Space: O(1)
class Solution {
public:
    int addDigits(int num) {
        return num ? (num % 9 ? num % 9 : 9) : 0;
    }
};

Or

// OJ: https://leetcode.com/problems/add-digits/
// Author: github.com/lzl124631x
// Time: O(1)
// Space: O(1)
class Solution {
public:
    int addDigits(int num) {
        return num ? 1 + (num - 1) % 9 : 0;
    }
};