class PseudoQueue:
"""Queue that uses two stacks"""
def __init__(self):
"""Method initializer
"""
self.add = Stack()
self.remove = Stack()
def __len__(self):
return len(self.add) + len(self.remove)
def enqueue(self, val):
"""Add a new element ot the queue
Args:
val (any): Value to be added
Returns:
any: Value that was added to the queue
"""
try:
while self.remove.peek():
self.add.push(self.remove.pop())
except AttributeError as err:
pass
self.add.push(val)
return self.add.top.val
def dequeue(self):
"""Remove the first element from the queue and return its value
Raises:
AttributeError: When the method is called on the empty queue
Returns:
any: Value of the removed element
"""
try:
while self.add.peek():
self.remove.push(self.add.pop())
except AttributeError as err:
pass
try:
return self.remove.pop()
except AttributeError as err:
raise AttributeError('Cannot be called on empty queue')
class PseudoQueue:
def __init__(self, stack1):
self.stack1 = Stack()
self.stack2 = Stack()
def enqueue(self, value):
self.stack1.push(value)
def dequeue(self):
if self.stack1.top == None:
raise EmptyQueueException()
while self.stack1.top != None:
popped_value = self.stack1.pop()
self.stack2.push(popped_value)
stack2_popped = self.stack2.pop()
while self.stack2.top != None:
self.stack1.push(self.stack2.pop())
return stack2_popped
def peek(self):
return self.stack1.peek()
def test_stack_peek():
value = 1
node = Node(value)
stack = Stack(node)
actual = stack.peek()
expected = value
assert actual == expected
def test_peek():
s = Stack()
s.push("apple")
s.push("banana")
actual = s.peek()
expected = "banana"
assert actual == expected
class AnimalShelter:
def __init__(self):
self.main_stack = Stack()
self.helper_stack = Stack()
def enqueue(self, animal):
while not isinstance(self.main_stack.peek(), EmptyStackException):
self.helper_stack.push(self.main_stack.pop())
self.helper_stack.push(animal)
while not isinstance(self.helper_stack.peek(), EmptyStackException):
self.main_stack.push(self.helper_stack.pop())
def dequeue(self, animal):
if isinstance(self.main_stack.peek(), EmptyStackException):
return EmptyStackException()
saved_animal = None
while not isinstance(self.main_stack.peek(), EmptyStackException):
if not saved_animal and isinstance(self.main_stack.peek(), animal):
saved_animal = self.main_stack.pop()
else:
self.helper_stack.push(self.main_stack.pop())
while not isinstance(self.helper_stack.peek(), EmptyStackException):
self.main_stack.push(self.helper_stack.pop())
return saved_animal
def peek(self):
return self.main_stack.peek()
def test_raise_exception_when_peeking():
stack = Stack()
with pytest.raises(InvalidOperationError) as e:
stack.peek()
assert str(e.value) == "Peeking from an empty stack!"
def test_peek():
stack = Stack()
stack.push(1)
actual = stack.peek()
expected = 1
assert actual == expected
def test_stack_peek_empty_exception():
stack = Stack()
with pytest.raises(Exception):
stack.peek()
def test_peek_empty():
s = Stack()
with pytest.raises(InvalidOperationError) as e:
s.peek()
assert str(e.value) == "Method not allowed on empty collection"
