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Sum of Digits - Implementation - Recursion Fundamentals | Dynamic Programming | Repovive

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Dynamic Programming21 sections · 918 units60h total

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Sum of Digits - Implementation

Complete solution

Here's the recursive solution:

function sum_of_digits(n)
    if n < 10 then
        return n
    return (n mod 10) + sum_of_digits(n / 10)

Try tracing sum_of_digits(1234) yourself. What's the first call? When do you stop? You compute $4 + \text{sum}(123)$ , which becomes $4 + 3 + \text{sum}(12)$ , then $4 + 3 + 2 + \text{sum}(1)$ , and finally $4 + 3 + 2 + 1 = 10$ .

Time complexity: $O(\log n)$ since you process one digit per call.

Space complexity: $O(\log n)$ for the call stack.