258. Add Digits – Leetcode Solutions

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

class Solution:
    def addDigits(self, num: int) -> int:
        # If the input number is 0, return 0 immediately as the sum of digits of 0 is 0
        if num == 0:
            return 0

        # Digital root formula:
        # If num % 9 == 0 and num is not zero, return 9 (since 9 is the digital root of numbers like 9, 18, 27, etc.).
        # Otherwise, return num % 9 which gives the digital root for all other cases.
        return 9 if num % 9 == 0 else num % 9

class Solution {
public:
    int addDigits(int num) {
        // If the input number is 0, return 0 immediately
        if (num == 0)
            return 0;

        // Using the digital root formula:
        // If num % 9 == 0 and num is not zero, return 9.
        // Otherwise, return num % 9. This formula works because:
        // - For any number, the digital root can be computed as the number mod 9.
        // - Special case: when num is divisible by 9 (i.e., num % 9 == 0), the digital root is 9,
        //   unless num itself is zero.
        
        // Return 9 if num is divisible by 9 and not zero, otherwise return num % 9.
        return (num % 9 == 0) ? 9 : (num % 9);
    }
};

var addDigits = function(num) {
    // If the input number is 0, return 0 immediately
    if (num === 0) {
        return 0;
    }

    // Using the digital root formula:
    // The digital root can be calculated using modulo 9.
    // If num is divisible by 9 and not zero, the result should be 9.
    // Otherwise, the result is num % 9.
    
    // Return 9 if num is divisible by 9 and not zero, otherwise return num % 9
    return (num % 9 === 0) ? 9 : (num % 9);
};

class Solution {
    public int addDigits(int num) {
        // If the input number is 0, return 0 immediately
        if (num == 0) {
            return 0;
        }

        // Using the digital root formula:
        // The digital root can be computed using modulo 9.
        // If num is divisible by 9 and not zero, the result should be 9.
        // Otherwise, the result is num % 9.
        
        // Check if num is divisible by 9
        if (num % 9 == 0) {
            return 9; // Return 9 for multiples of 9
        } else {
            return num % 9; // Return the remainder for other cases
        }
    }
}