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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0201_0300.s0258_add_digits;

// #Easy #Math #Simulation #Number_Theory #2022_07_05_Time_1_ms_(100.00%)_Space_39.3_MB_(98.44%)

/**
 * 258 - Add Digits\.
 *
 * Easy
 *
 * Given an integer `num`, repeatedly add all its digits until the result has only one digit, and return it.
 *
 * **Example 1:**
 *
 * **Input:** num = 38
 *
 * **Output:** 2
 *
 * **Explanation:**
 *
 *     The process is
 *     38 --> 3 + 8 --> 11
 *     11 --> 1 + 1 --> 2
 *     Since 2 has only one digit, return it. 
 *
 * **Example 2:**
 *
 * **Input:** num = 0
 *
 * **Output:** 0 
 *
 * **Constraints:**
 *
 * *   0 <= num <= 2³¹ - 1
 *
 * **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;
        }
        return num % 9;
    }
}