A parcel service stamps a one-digit checksum on each label: add up the digits of the tracking number; if the result has more than one digit, add up its digits again; repeat until a single digit remains. Write checksum_digit(n) returning that final digit for a non-negative integer n, computing every digit sum recursively (no string conversion, no loops).
Examples
Input: n = 493193
Output: 2
Explanation: 4+9+3+1+9+3 = 29, then 2+9 = 11, then 1+1 = 2.
Input: n = 7
Output: 7
Constraints
0 <= n < 10**18
Goals
- Compose two recursive functions: one for the digit sum, one that repeats it
- Stop when the value becomes a single digit
- Handle 0 without an infinite loop