258. Add Digits

Problem

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?

Solution

/**
 * @param {number} num
 * @return {number}
 */
var addDigits = function(num) {
    var res = 0;
    while (num) {
        res += num % 10;
        num = Math.floor(num / 10);
    }
    return res < 10 ? res : addDigits(res);
};

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(1).

Solution

/**
 * @param {number} num
 * @return {number}
 */
var addDigits = function(num) {
  return 1 + (num - 1) % 9;
};

Explain:

结果只有一个数字，就是 1~9 ，不断循环，找规律即可。

Complexity:

• Time complexity : O(1).
• Space complexity : O(1).