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)