Added Stack using Linked Lists.

Author: shreyasvedpathak

README.md

@@ -2,23 +2,29 @@
 
 ## Algorithms
 
-### Searching Algorithms
+### 1. Searching Algorithms
 
 - [Binary Search - both Recursive and Iterative Approach](Searching%20Algorithm/binarysearch.py)
 
-### Sorting Algorithms
+### 2. Sorting Algorithms
 
-#### [Index](Sorting%20Algorithms/README.md)
+Find the detailed description on [Index](Sorting%20Algorithms/) page.
 
 - [Sorting Algorithms Code](Sorting%20Algorithms/sortingAlgo.py)
 - [Random Numbers for Input](Sorting%20Algorithms/num.txt)
 
 ## Data Structures
 
-### Linked Lists
+### 1. Linked Lists
 
 Find the detailed description on operations performed [here.](Linked%20List/)
 
 1. [Linear Linked List](Linked%20List/linkedlist.py)
 2. [Circular Linked List](Linked%20List/circularLL.py)
 3. [Doubly Linked List](Linked%20List/doublyLL.py)
+
+### 2. Stack
+
+1. [Stack using inbuilt Python List](Stacks/stack.py)
+2. [Stack using Linked List](Stacks/stackLL.py)
+3. 

Stacks/stacks.py renamed to Stacks/stack.py

@@ -0,0 +1,150 @@
+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 StackLL:
+    '''
+    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 stack.
+        '''
+        return self._size
+
+    def isempty(self):
+        '''
+        Returns True if stack is empty, otherwise False.
+        '''
+        return self._size == 0
+
+    def push(self, e):
+        '''
+        Pushes the passed element at the beginning of the linked list.
+        That means, on the top of our stack.
+        '''
+        newest = _Node(e, None)
+
+        if self.isempty():
+            self._head = newest
+            self._tail = newest
+        else:
+            newest._link = self._head
+            self._head = newest
+        self._size += 1
+
+    def pop(self):
+        '''
+        Removes element from the beginning of the linked list.
+        That means, pops element that is on the top.
+        Returns the removed element.
+        '''
+        if self.isempty():
+            print("Stack is Empty. Cannot perform Pop 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 top(self):
+        '''
+        Peeks at the element on the top of the stack.
+        '''
+        if self.isempty():
+            print("Stack is Empty. Cannot perform Pop operation.")
+            return
+
+        e = self._head._element
+        return e
+
+    def display(self):
+        '''
+        Utility function to display the Stack.
+        '''
+        if self.isempty() == 0:
+            p = self._head
+            while p:
+                print(p._element)
+                p = p._link
+        else:
+            print("Empty")
+
+###############################################################################
+
+def options():
+    '''
+    Prints Menu for operations
+    '''
+    options_list = ['Push', 'Pop', 'Top',
+                    'Display Stack', '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: "))
+        S.push(elem)
+
+    elif choice == 2:
+        print('Popped item is: ', S.pop())
+
+    elif choice == 3:
+        print("Item on top is: ", S.top())
+
+    elif choice == 4:
+        print("Stack:")
+        S.display()
+        print("\n")
+
+    elif choice == 5:
+        import sys
+        sys.exit()
+
+###############################################################################
+
+
+S = StackLL()
+while True:
+    choice = options()
+    switch_case(choice)