Compute the sum of n’s decimal digits recursively – the same problem as Module 1’s iterative “Digit Sum”
exercise, solved with a different technique.
The base case is a single-digit number (n < 10), which is its own digit sum. Otherwise, split n into its last
digit (n % 10) and everything before it (n / 10, exactly as before), and let a recursive call handle “everything
before it” while you add the last digit on. Notice the base case here isn’t “n reaches exactly 0” the way the
while version’s loop condition was – it’s “n is already a single digit,” a subtly different (and for this
problem, more natural) stopping point.