What is a stack?

A stack is a collection where you can only add or remove items at one end, called the top. The last item you put in is always the first one you take out. That rule has a name: LIFO — last in, first out.

A stack of four plates. A new plate is pushed onto the top, and the only plate that can be taken off is the top one. The plates underneath can't be reached without removing the ones above them. — Only the top is ever touched.

Think of a stack of plates

In a cafeteria, clean plates are stacked on top of each other. You put a clean plate on top, and you take the plate from the top. Nobody pulls a plate out of the middle. A stack in programming follows exactly the same rules:

push — put a new item on top.
pop — take the top item off (and get it back).
peek (also called top) — look at the top item without removing it.
isEmpty — are there any items at all?
size — how many items are there?

Every one of these is O(1): none of them ever has to look past the top item, no matter how tall the stack gets.

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Last in, first out
A stack in code
Stack or queue?
Underflow and overflow
One idea, two ways to build it
Words you'll see everywhere
Key takeaways

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What is a stack?

Think of a stack of plates

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