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108 lines (74 loc) · 3.18 KB
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"""This module holds classes related to LIFO stacks.
This module contains `Stack`, an implementation of a LIFO stack, and `StackUnderflowError`, an error raised by `Stack`.
Examples:
Initializing, adding, and removing elements from a `Stack`:
s = Stack()
s.push(1)
s.push(2)
s.pop()
The following code will raise a `StackUnderflowError` because the second pop is done on an empty `Stack`:
s.pop()
s.pop()
"""
from collections import deque
class StackUnderflowError(Exception):
"""This class is used by `Stack` to raise errors for operations done on an empty `Stack`."""
def __init__(self, operation):
"""Initializes a `StackUnderflowError` that will be raised associated with a particular `Stack` operation.
Args:
operation: a string specifying the operation to raise an error on
"""
super().__init__(f'Cannot perform {operation} on an empty stack.')
class Stack:
"""This class represents a LIFO stack that has no maximum capacity.
Examples:
To initialize a `Stack`:
s = Stack()
To add elements to the end of `s`:
s.push(1)
s.push(2)
To remove and return the element at the top of `s` (in this case `x = 2`):
x = s.pop()
To see the element at the top of `s` (in this case `y = 1`):
y = s.top()
"""
def __init__(self):
"""Initializes an empty `Stack` in `O(1)` time."""
self.__buf = deque()
def top(self):
"""Gets the element at the top of this `Stack`.
This operation runs in `O(1)` time with respect to the size of this `Stack`.
Returns:
The element at the top of the `Stack`. That is, the element that was last added to this `Stack` of the
elements in it.
Raises:
StackUnderflowError: If this `Stack` is empty.
"""
if len(self.__buf) == 0:
raise StackUnderflowError('top()')
return self.__buf[-1]
def pop(self):
"""Removes the element at the top of this `Stack`.
This operation runs in `O(1)` time with respect to the size of this `Stack`.
Returns:
The element at the top of the `Stack` that was removed. That is, the element that was last added to this
`Stack` of the elements in it.
Raises:
StackUnderflowError: If this `Stack` is empty.
"""
if len(self.__buf) == 0:
raise StackUnderflowError('pop()')
return self.__buf.pop()
def push(self, value):
"""Adds an element to the top of this `Stack`.
This operation runs in `O(1)` time with respect to the size of this `Stack`.
Args:
value: Element to add to this `Stack`. It can be of any type.
"""
self.__buf.append(value)
def is_empty(self):
"""Returns `True` if this `Stack` is empty, `False` otherwise in `O(1)` time w/r/t the size of this `Stack`."""
return len(self.__buf) == 0
def size(self):
"""Returns the integer number of elements in this `Stack` in `O(1)` time w/r/t the size of this `Stack`."""
return len(self.__buf)