Given a non-negative integer n, return the sum of its digits. The loopExtraction LoopRepeatedly reading the last digit with % 10, adding it to a running total, then removing it with integer division by 10. version is the direct, iterative way to peel digits off one at a time. The recursiveRecursionsumOfDigits(n) = n's last digit + sumOfDigits of everything before it — the same problem, one digit smaller each call. version expresses the exact same idea as "this digit, plus the digit sum of the rest," with 0 as the natural base case.