Stack class

LIFO with exception on underflow.

OOP stack data-structures

Implement class `Stack` with:
- `push(val)` — add to top
- `pop() -> any` — remove and return top; raise `IndexError` if empty
- `peek() -> any` — return top without removing; raise `IndexError` if empty
- `is_empty() -> bool`
- `size() -> int`

Constraints

Any hashable values.

Example

s = Stack()
s.push(1); s.push(2)
s.pop()  # 2
s.peek() # 1

12 points ~15 min

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Hints

Use a list internally; pop() maps to list.pop().

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