Stack is empty — push a value
AlgoPlus//structures / stack
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Stack · LIFO

Last-In-First-Out — push and pop happen at the top.

Stability
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In-Place
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Space Complexity
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Avg Time
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Legend
Push
Pop
Top / peek
AI Tutor Workspace
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In a nutshell
A stack is a pile where you only ever touch the top: you push a value on, pop the top one off, or peek at it without removing it. That makes it Last-In-First-Out — the most recent thing added is the first to leave, like a stack of plates. Each operation is instant, and this simple rule is exactly what powers undo buttons, the browser back button, and the call stack that tracks running functions.
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Like a pile of plates — you add and take only from the top, so the last thing in is the first thing out.
Key terms
Go deeper in the lesson
Read the full theory, intuition & complexity for Stack · LIFO.
Code Simulator
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class Stack:
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    def __init__(self):
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        self.items = []
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    def push(self, item):
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        self.items.append(item)  # Push
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    def pop(self):
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        if not self.is_empty():
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            return self.items.pop()  # Pop
Why Python? · Readable first, fast second

Dynamically typed and interpreted — every comparison and swap is dispatched by the interpreter at run time, so tight loops run roughly 10–100× slower than compiled C/C++. Unbeatable for learning the idea with the least code; not what you reach for when the inner loop is the bottleneck.