Given a non-negative integer, repeatedly sum its digits until the result is a single digit (0-9). This is also known as the digital root of a number.
For example, given 38:
- First iteration: 3 + 8 = 11
- Second iteration: 1 + 1 = 2
- Result: 2 (single digit achieved)
Method Signature:
public static Integer digitalRoot(Integer num)
Examples:
Example 1:
Input: 38
Output: 2
Explanation: 3 + 8 = 11, then 1 + 1 = 2
Example 2:
Input: 123
Output: 6
Explanation: 1 + 2 + 3 = 6 (already single digit)
Example 3:
Input: 9
Output: 9
Explanation: Already a single digit
Example 4:
Input: 0
Output: 0
Explanation: Zero is a single digit
Example 5:
Input: null
Output: null
Explanation: Handle null input gracefully
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