DSA · Theory

Stack

Theory

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Don’t start Stack with jargon. Start with undo typing. That is the whole point.

For Stack, a dry-run table is in play. Don’t blame it until you have traced undo typing.

Stack exists so Riya can keep undo typing correct in a tiny editor.

Stack shows up in a tiny editor. Name undo typing, not “a real-world scenario”.

If undo typing breaks under Stack, check pop on empty first.

Place Stack next to nearby DSA work — push / pop LIFO is the link.

Close Stack with — “If I skip it, undo typing goes wrong like this: pop on empty.”

Diagram

    | 3 |  ← top (pop)
    | 2 |
    | 1 |
    LIFO

Exam tip

Say Stack in one breath, then push / pop LIFO, then pop on empty.

stack = []
stack.append(10)   # push
stack.append(20)
print(stack.pop()) # 20
print(stack[-1])   # peek → 10

Stack is LIFO: push/pop from the same end.