Python/data_structures/stacks/stack_using_dll.py

# A complete working Python program to demonstrate all  
# stack operations using a doubly linked list  
  
class Node: 
    def __init__(self, data): 
        self.data = data # Assign data 
        self.next = None # Initialize next as null 
        self.prev = None # Initialize prev as null         
          
class Stack:
    """
    >>> stack = Stack()
    >>> stack.is_empty()
    True
    >>> stack.print_stack()
    stack elements are:
    >>> for i in range(4):
    ...     stack.push(i)
    ...
    >>> stack.is_empty()
    False
    >>> stack.print_stack()
    stack elements are:
    3->2->1->0->
    >>> stack.top()
    3
    >>> len(stack)
    4
    >>> stack.pop()
    3
    >>> stack.print_stack()
    stack elements are:
    2->1->0->
    """
    def __init__(self): 
        self.head = None
          
    def push(self, data):
        """add a Node to the stack"""
        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             
              
    def pop(self):
        """pop the top element off the stack"""
        if self.head is None: 
            return None
        else: 
            temp = self.head.data 
            self.head = self.head.next
            self.head.prev = None
            return temp 
  
    def top(self):
        """return the top element of the stack"""
        return self.head.data

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

    def is_empty(self):
        return self.head is None

    def print_stack(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.print_stack() 
  
    # Print the top element 
    print("\nTop element is ", stack.top()) 
  
    # Print the stack size 
    print("Size of the stack is ", len(stack)) 
  
    # pop the top element 
    stack.pop() 
  
    # pop the top element 
    stack.pop() 
    
    # two elements have now been popped off
    stack.print_stack() 
    
    # Print True if the stack is empty else False 
    print("\nstack is empty:", stack.is_empty())