creating mergeable stack

class Node():

def __init__(self,data):

self.next = None

self.prev = None

self.data = data

class Stack():

def __init__(self):

self.head = None

self.tail = None

def push(self, data):

new_node = Node(data)

if (self.head == None):

self.head = new_node

self.head.next= None

self.head.prev = None

self.tail = new_node

else:

new_node.prev = self.tail

self.tail.next = new_node

self.tail = new_node

def pop(self):

if (self.head == None):

print("Stack underflow")

if (self.head == self.tail):

self.head = None

self.tail = None

else:

node = self.tail

self.tail = self.tail.prev

del node

self.tail.next = None

def merge(self, stack):

if stack.head == None: return 

if self.head == None:

self.head = stack.head

self.tail = stack.tail

return

self.head.prev = stack.tail 

stack.tail.nxt = self.head

self.head = stack.head

def display(self):

if (self.tail != None):

n = self.tail

while (n != None):

print(n.data, end = " ")

n = n.prev

print()

else:

print("Stack Underflow")

ms1 = Stack()

ms2 = Stack()

ms1.push(6)

ms1.push(5)

ms1.push(4)

ms2.push(9)

ms2.push(8)

ms2.push(7)

ms1.merge(ms2)

ms1.display()

while ms1.head != ms1.tail:

ms1.pop ()

print ("check pop all elements until head == tail (one element left)")

print ("on merged stack: ", end = "")

ms1.display()