Stack using double linked list (#1413)

* Stack using double linked list

* Test with doctests

* Update stack_using_dll.py

* Update stack_using_dll.py

* Update stack_using_dll.py

Co-authored-by: Christian Clauss <cclauss@me.com>

data_structures/stacks/stack_using_dll.py

@@ -0,0 +1,120 @@
+# 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())