Exercise 1: Hand-Tracing Factorial(4) — Possible Solution
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THE FULL CALL CHAIN, DOWN TO THE BASE CASE
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Factorial(4): n is not 0, so call Factorial(3) first
  Factorial(3): n is not 0, so call Factorial(2) first
    Factorial(2): n is not 0, so call Factorial(1) first
      Factorial(1): n is not 0, so call Factorial(0) first
        Factorial(0): n = 0, BASE CASE, return 1

EVERY MULTIPLICATION AS THE RESULTS COMBINE BACK UP
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        Factorial(0) returns 1
      Factorial(1) returns 1 * 1 = 1
    Factorial(2) returns 2 * 1 = 2
  Factorial(3) returns 3 * 2 = 6
Factorial(4) returns 4 * 6 = 24

FINAL ANSWER: 24

WHY THIS MATCHES 4! CALCULATED THE ORDINARY WAY
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4! = 4 * 3 * 2 * 1 = 24, confirming the recursive result. The recursive
version arrives at the same multiplication, just built up from the
smallest factor outward (starting at the base case, 0! = 1, then
multiplying in 1, then 2, then 3, then 4) rather than the more familiar
top-down order.

WHY THIS WORKS AS AN ANSWER
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The trace shows every single recursive call made, all the way down to
the base case, and every multiplication performed as each call's result
returns back up to its caller - following this chapter's own worked
Factorial(5) trace structure exactly, one level shallower.