Python/data_structures/queue/queue_by_two_stacks.py

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"""Queue implementation using two stacks"""
from collections.abc import Iterable
from typing import Generic, TypeVar
_T = TypeVar("_T")
class QueueByTwoStacks(Generic[_T]):
def __init__(self, iterable: Iterable[_T] | None = None) -> None:
"""
>>> QueueByTwoStacks()
Queue(())
>>> QueueByTwoStacks([10, 20, 30])
Queue((10, 20, 30))
>>> QueueByTwoStacks((i**2 for i in range(1, 4)))
Queue((1, 4, 9))
"""
self._stack1: list[_T] = list(iterable or [])
self._stack2: list[_T] = []
def __len__(self) -> int:
"""
>>> len(QueueByTwoStacks())
0
>>> from string import ascii_lowercase
>>> len(QueueByTwoStacks(ascii_lowercase))
26
>>> queue = QueueByTwoStacks()
>>> for i in range(1, 11):
... queue.put(i)
...
>>> len(queue)
10
>>> for i in range(2):
... queue.get()
1
2
>>> len(queue)
8
"""
return len(self._stack1) + len(self._stack2)
def __repr__(self) -> str:
"""
>>> queue = QueueByTwoStacks()
>>> queue
Queue(())
>>> str(queue)
'Queue(())'
>>> queue.put(10)
>>> queue
Queue((10,))
>>> queue.put(20)
>>> queue.put(30)
>>> queue
Queue((10, 20, 30))
"""
return f"Queue({tuple(self._stack2[::-1] + self._stack1)})"
def put(self, item: _T) -> None:
"""
Put `item` into the Queue
>>> queue = QueueByTwoStacks()
>>> queue.put(10)
>>> queue.put(20)
>>> len(queue)
2
>>> queue
Queue((10, 20))
"""
self._stack1.append(item)
def get(self) -> _T:
"""
Get `item` from the Queue
>>> queue = QueueByTwoStacks((10, 20, 30))
>>> queue.get()
10
>>> queue.put(40)
>>> queue.get()
20
>>> queue.get()
30
>>> len(queue)
1
>>> queue.get()
40
>>> queue.get()
Traceback (most recent call last):
...
IndexError: Queue is empty
"""
# To reduce number of attribute look-ups in `while` loop.
stack1_pop = self._stack1.pop
stack2_append = self._stack2.append
if not self._stack2:
while self._stack1:
stack2_append(stack1_pop())
if not self._stack2:
raise IndexError("Queue is empty")
return self._stack2.pop()
if __name__ == "__main__":
from doctest import testmod
testmod()