Stack
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Stack is a data structure that allows to store and retrieve elements in a last-in-first-out (LIFO) order.
The operations of a stack are:

- `push`: add an element to the top of the stack
- `pop`: remove the top element from the stack
- `peek`: return the top element of the stack without removing it
- `is_empty`: check if the stack is empty

Here are collected selected problems that use stacks as a data structure.

Valid Parentheses
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.. autofunction:: stack.stack.is_valid