Added Queue and QueueLL code.

Author: shreyasvedpathak

Queues/queues.py

@@ -0,0 +1,109 @@
+import os
+
+
+class Queue:
+def __init__(self):
+self._data = []
+
+def __len__(self):
+'''
+Returns length of Queue
+'''
+return len(self._data)
+
+def isempty(self):
+'''
+Returns True if Queue is empty, otherwise False.
+'''
+return self.__len__ == 0
+
+def enqueue(self, e):
+'''
+Adds the passed element at the end of the list.
+That means, it enqueues element.
+'''
+self._data.append(e)
+
+def dequeue(self):
+'''
+Removes element from the beginning of the list and
+returns the removed element.
+That means, it dequeues.
+'''
+if self.isempty():
+print("Queue is empty.")
+return
+
+return self._data.pop(0)
+
+def first(self):
+'''
+Peeks and return the first element in the Queue.
+'''
+if self.isempty():
+print("Queue is empty.")
+return
+
+return self._data[0]
+
+def display(self):
+'''
+Utility function to display the Queue.
+'''
+if self.isempty():
+print("Queue is empty.")
+else:
+print("Front", end=' <--')
+for i in self._data:
+print(i, end='<--')
+print(" Rear")
+
+###############################################################################
+
+
+def options():
+'''
+Prints Menu for operations
+'''
+options_list = ['Enqueue', 'Dequeue', 'First',
+'Display Queue', 'Exit']
+
+print("MENU")
+for i, option in enumerate(options_list):
+print(f'{i + 1}. {option}')
+
+choice = int(input("Enter choice: "))
+return choice
+
+
+def switch_case(choice):
+'''
+Switch Case for operations
+'''
+os.system('cls')
+if choice == 1:
+elem = int(input("Enter item to Enqueue: "))
+Q.enqueue(elem)
+
+elif choice == 2:
+print('Dequeued item is: ', Q.dequeue())
+
+elif choice == 3:
+print("First item is: ", Q.first())
+
+elif choice == 4:
+print("Queue: ")
+Q.display()
+print("\n")
+
+elif choice == 5:
+import sys
+sys.exit()
+
+###############################################################################
+
+
+Q = Queue()
+while True:
+choice = options()
+switch_case(choice)

Queues/queuesLL.py

@@ -0,0 +1,151 @@
+import os
+
+
+class _Node:
+'''
+Creates a Node with two fields:
+1. element (accesed using ._element)
+2. link (accesed using ._link)
+'''
+__slots__ = '_element', '_link'
+
+def __init__(self, element, link):
+'''
+Initialses _element and _link with element and link respectively.
+'''
+self._element = element
+self._link = link
+
+
+class QueueLL:
+'''
+Consists of member funtions to perform different
+operations on the linked list.
+'''
+
+def __init__(self):
+'''
+Initialses head, tail and size with None, None and 0 respectively.
+'''
+self._head = None
+self._tail = None
+self._size = 0
+
+def __len__(self):
+'''
+Returns length of Queue
+'''
+return self._size
+
+def isempty(self):
+'''
+Returns True if Queue is empty, otherwise False.
+'''
+return self._size == 0
+
+def enqueue(self, e):
+'''
+Adds the passed element at the end of the linked list.
+That means, it enqueues element.
+'''
+newest = _Node(e, None)
+
+if self.isempty():
+self._head = newest
+else:
+self._tail._link = newest
+
+self._tail = newest
+self._size += 1
+
+def dequeue(self):
+'''
+Removes element from the beginning of the linked list and
+returns the removed element.
+That means, it dequeues.
+'''
+if self.isempty():
+print("Queue is Empty. Cannot perform dequeue operation.")
+return
+
+e = self._head._element
+self._head = self._head._link
+self._size = self._size - 1
+
+if self.isempty():
+self._tail = None
+return e
+
+def first(self):
+'''
+Peeks and return the first element in the Queue.
+'''
+if self.isempty():
+print("Queue is Empty.")
+return
+e = self._head._element
+return e
+
+def display(self):
+'''
+Utility function to display the Queue.
+'''
+if self.isempty() == 0:
+p = self._head
+print("Front", end=' <--')
+while p:
+print(p._element, end='<--')
+p = p._link
+print(" Rear")
+else:
+print("Empty")
+
+###############################################################################
+
+
+def options():
+'''
+Prints Menu for operations
+'''
+options_list = ['Enqueue', 'Dequeue', 'First',
+'Display Queue', 'Exit']
+
+print("MENU")
+for i, option in enumerate(options_list):
+print(f'{i + 1}. {option}')
+
+choice = int(input("Enter choice: "))
+return choice
+
+
+def switch_case(choice):
+'''
+Switch Case for operations
+'''
+os.system('cls')
+if choice == 1:
+elem = int(input("Enter item to Enqueue: "))
+Q.enqueue(elem)
+
+elif choice == 2:
+print('Dequeued item is: ', Q.dequeue())
+
+elif choice == 3:
+print("First item is: ", Q.first())
+
+elif choice == 4:
+print("Queue: ")
+Q.display()
+print("\n")
+
+elif choice == 5:
+import sys
+sys.exit()
+
+###############################################################################
+
+
+Q = QueueLL()
+while True:
+choice = options()
+switch_case(choice)

README.md

@@ -27,3 +27,8 @@ Find the detailed description on operations performed [here.](Linked%20List/)
 
 1. [Stack using inbuilt Python List](Stacks/stack.py)
 2. [Stack using Linked List](Stacks/stackLL.py)
+
+### 3. Queue
+
+1. [Queue using inbuilt Python List](Queues/queues.py)
+2. [Queue using Linked List](Queues/queuesLL.py)