Repeated Digit Sum
Take a non-negative integer n. Replace it with the sum of its decimal digits, and keep doing so until a single digit remains. Return that digit.
Can you find an O(1) formula?
Example 1
Input: n = 9875 Output: 2 Explanation: 9+8+7+5 = 29, 2+9 = 11, 1+1 = 2.
Example 2
Input: n = 0 Output: 0
Constraints
- 0 <= n <= 2^31 - 1
Hint 1
A number and the sum of its digits leave the same remainder when divided by 9.
Hint 2
Handle 0 and multiples of 9 carefully.
Good practice for: Frontend Developer, Backend Developer, Full Stack Developer, Mobile App Developer, Game Developer, Automation Test Engineer, Embedded Systems Engineer
Run checks your code on 2 sample tests. Submit runs all 7 tests, including hidden ones, and earns points. Use console.log to debug. Python's first run can take a few seconds while it starts.