Trace lab in Stacks and Expression Processing

Mission

Push tokens then pop them in reverse order.

Learning outcome: Trace LIFO push and pop order

Correctness contract

Invariant: Only the top element is removed, and size equals pushes minus successful pops.

Required technique: Push every token onto a LIFO stack, then pop to produce the reverse order.

Complexity target: time O(n); space O(n).

Input and output

Input: n followed by exactly n whitespace-delimited tokens. Whitespace may be spaces or line breaks.

Output: Print the requested sequence on one line with single spaces and no trailing space. Return it as a String; Main.java prints it without adding other text.

Assumptions:

• The declared count is non-negative and is followed by exactly that many whitespace-delimited tokens.

Before you code

1. Restate the input and output contract, then predict the visible example without running code.
2. Implement the core state transition: Push every token onto a LIFO stack, then pop to produce the reverse order.
3. Trace the smallest boundary case, verify exact formatting, and justify the authored time and auxiliary-space bounds.

Implement Practice.solve(Scanner sc). Keep every provided filename and public class name unchanged.

Sample input

4 one two three four

Sample output

four three two one

Why the sample works: Visible walkthrough for the ordinary non-trivial path. Tokens leave the stack in the reverse of their push order. Input `4 one two three four` therefore produces `four three two one`.

Progressive hints

Try the trace and first milestone before opening a hint. Open them in order.

Hint 1 — Contract: identify what each parsed variable represents and write the invariant beside the loop or recursive method.

Hint 2 — Next step: Trace the smallest non-trivial input and write the structure state after the operation before coding the loop.

Hint 3 — Verification: compare the structure state before and after one operation, then test the smallest valid input and a duplicate or unreachable case when allowed.