Create queue_by_two_stacks.py

data_structures/queue/queue_by_two_stacks.py

@@ -0,0 +1,115 @@
+"""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()