Python | Stack using Doubly Linked List

A stack is a collection of objects that are inserted and removed using Last in First out Principle(LIFO). User can insert elements into the stack, and can only access or remove the recently inserted object on top of the stack. The main advantage of using LinkedList over array for implementing stack is the dynamic allocation of data whereas in the array, the size of the stack is restricted and there is a chance of stack overflow error when the size of the stack is exceeded the maximum size.

Stack Operations:

1. push() : Insert the element into Stack and assign the top pointer to the element.
2. pop() : Return top element from the Stack and move the top pointer to the second element of the Stack.
3. top() : Return the top element.
4. size() : Return the Size of the Stack.
5. isEmpty() : Return True if Stack is Empty else return False.
6. printstack() : Print all elements of the stack.

Below is the implementation of the above-mentioned stack operations using Doubly LinkedList in Python:

Implementation of Stack using Doubly Linked List 

Python3

# A complete working Python program to demonstrate all
# stack operations using a doubly linked list

# Node class
class Node:

# Function to initialise the node object
    def __init__(self, data):
        self.data = data # Assign data
        self.next = None # Initialize next as null
        self.prev = None # Initialize prev as null

# Stack class contains a Node object
class Stack:
    # Function to initialize head
    def __init__(self):
        self.head = None

# Function to add an element data in the stack
    def push(self, data):

        if self.head is None:
            self.head = Node(data)
        else:
            new_node = Node(data)
            self.head.prev = new_node
            new_node.next = self.head
            new_node.prev = None
            self.head = new_node


# Function to pop top element and return the element from the stack
    def pop(self):

        if self.head is None:
            return None
        elif self.head.next is None:
            temp = self.head.data
            self.head = None
            return temp
        else:
            temp = self.head.data
            self.head = self.head.next
            self.head.prev = None
            return temp




# Function to return top element in the stack
    def top(self):

        return self.head.data


# Function to return the size of the stack
    def size(self):

        temp = self.head
        count = 0
        while temp is not None:
            count = count + 1
            temp = temp.next
        return count


# Function to check if the stack is empty or not
    def isEmpty(self):

        if self.head is None:
           return True
        else:
           return False


# Function to print the stack
    def printstack(self):

        print("stack elements are:")
        temp = self.head
        while temp is not None:
            print(temp.data, end ="->")
            temp = temp.next


# Code execution starts here
if __name__=='__main__':

# Start with the empty stack
  stack = Stack()

# Insert 4 at the beginning. So stack becomes 4->None
  print("Stack operations using Doubly LinkedList")
  stack.push(4)

# Insert 5 at the beginning. So stack becomes 4->5->None
  stack.push(5)

# Insert 6 at the beginning. So stack becomes 4->5->6->None
  stack.push(6)

# Insert 7 at the beginning. So stack becomes 4->5->6->7->None
  stack.push(7)

# Print the stack
  stack.printstack()

# Print the top element
  print("\nTop element is ", stack.top())

# Print the stack size
  print("Size of the stack is ", stack.size())

# pop the top element
  stack.pop()

# pop the top element
  stack.pop()

# two elements are popped
# Print the stack
  stack.printstack()

# Print True if the stack is empty else False
  print("\nstack is empty:", stack.isEmpty())

#This code is added by Suparna Raut

Output:

Stack operations using Doubly LinkedList
stack elements are:
7->6->5->4->
Top element is  7
Size of the stack is  4
stack elements are:
5->4->
stack is empty: False

Time Complexity for operations:

• Push(): O(1)
• pop(): O(1)
• top(): O(1)
• size(): O(N)
• isEmpty(): O(1)
• printStack(): O(N)

Auxiliary Space for operations:

• Push(): O(1)
• pop(): O(1)
• top(): O(1)
• size(): O(1)
• isEmpty(): O(1)
• printStack(): O(1)