Recursion

Every recursive function needs a base case (when to stop) and a recursive case that moves toward it.

def factorial(n):
    if n <= 1:          # base case
        return 1
    return n * factorial(n - 1)

Each call uses a new stack frame, so very deep recursion can cause a stack overflow — a real risk on microcontrollers with only a few KB of RAM. Many recursive algorithms can be rewritten as loops.

Recursion shines for naturally recursive data: trees, divide-and-conquer sorting (merge sort, quick sort) and backtracking.