Study guides & original practice
Stack versus queue practice
A stack removes the most recently added item first. A queue removes the earliest waiting item first. Choose the structure by the required access order.
By ExamCat · Original study lesson · Published September 24, 2026
Understand the method
The common names are LIFO for stacks and FIFO for queues. These describe behavior, not a specific programming-language implementation. Real systems may use priority queues or other policies, which are different from a simple FIFO queue.
Worked example, step by step
Add A, then B, then C to an initially empty stack and an initially empty queue.
- The stack top is C, so its first removal returns C.
- The queue front is A, so its first removal returns A.
- Removing all items gives C, B, A for the stack.
- Removing all items gives A, B, C for the queue. The insertion order was identical; the removal policy changed.
A mistake to watch for
A queue is not necessarily a priority ranking. In a priority queue, urgency or another key can override arrival order. State the policy before predicting which task runs next.
Try it yourself: three questions
Write an answer before opening the explanation. For calculations, keep your units and intermediate steps; for concepts, state the reason for your choice.
1. Push 1, push 2, pop, push 3, pop on a stack. What do the pops return?
Show answer and explanation
First 2, then 3. The value 1 remains after these operations.
2. Enqueue 1, enqueue 2, dequeue, enqueue 3, dequeue. What returns?
Show answer and explanation
First 1, then 2. The value 3 remains in the queue.
3. Why is an undo history often modeled with a stack?
Show answer and explanation
The latest action is usually the first one reversed. More complicated undo/redo behavior may use additional structures.
Build your next practice session
Mark the first step where your reasoning differed from the explanation. Turn that step into a question, answer it without looking, then change one condition in the example and explain what would change. A correct guess is a reason to revisit the method.
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Further reading
OpenStax: data structures and algorithms provides textbook background. The worked scenario and practice questions on this page were written independently for ExamCat.
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