Add a __len__() function to doubly_linked_list.py

data_structures/linked_list/doubly_linked_list.py

@@ -5,45 +5,60 @@
 - Each link references the next link and the previous one.
 - A Doubly Linked List (DLL) contains an extra pointer, typically called previous
     pointer, together with next pointer and data which are there in singly linked list.
- - Advantages over SLL - IT can be traversed in both forward and backward direction.,
-     Delete operation is more efficient"""
+- Advantages over SLL - IT can be traversed in both forward and backward direction.
+    Delete operation is more efficient.
+"""
+
+
+class Link:
+    next = None  # This points to the link in front of the new link
+    previous = None  # This points to the link behind the new link
+
+    def __init__(self, x):
+        self.value = x
+
+    def displayLink(self):
+        print(self.value, end=" ")
 
 
-class LinkedList:  # making main class named linked list
+class LinkedList:
     def __init__(self):
         self.head = None
         self.tail = None
+        self.size = 0
 
-    def insertHead(self, x):
-        newLink = Link(x)  # Create a new link with a value attached to it
-        if self.isEmpty():  # Set the first element added to be the tail
-            self.tail = newLink
+    def insert_head(self, x):
+        new_link = Link(x)  # Create a new link with a value attached to it
+        if self.is_empty():  # Set the first element added to be the tail
+            self.tail = new_link
         else:
-            self.head.previous = newLink  # newLink <-- currenthead(head)
-        newLink.next = self.head  # newLink <--> currenthead(head)
-        self.head = newLink  # newLink(head) <--> oldhead
+            self.head.previous = new_link  # new_link <-- currenthead(head)
+        new_link.next = self.head  # new_link <--> currenthead(head)
+        self.head = new_link  # new_link(head) <--> oldhead
+        self.size += 1
 
-    def deleteHead(self):
+    def delete_head(self):
         temp = self.head
         self.head = self.head.next  # oldHead <--> 2ndElement(head)
-        # oldHead --> 2ndElement(head) nothing pointing at it so the old head will be
-        # removed
-        self.head.previous = None
-        if self.head is None:
+        self.head.previous = None  # oldHead --> 2ndElement(head) nothing pointing at it so the old head will be removed
+        if not self.head:
             self.tail = None  # if empty linked list
+        self.size -= 1
         return temp
 
-    def insertTail(self, x):
+    def insert_tail(self, x):
         newLink = Link(x)
         newLink.next = None  # currentTail(tail)    newLink -->
         self.tail.next = newLink  # currentTail(tail) --> newLink -->
         newLink.previous = self.tail  # currentTail(tail) <--> newLink -->
         self.tail = newLink  # oldTail <--> newLink(tail) -->
+        self.size += 1
 
-    def deleteTail(self):
+    def delete_tail(self):
         temp = self.tail
         self.tail = self.tail.previous  # 2ndLast(tail) <--> oldTail --> None
         self.tail.next = None  # 2ndlast(tail) --> None
+        self.size -= 1
         return temp
 
     def delete(self, x):
@@ -54,31 +69,39 @@ def delete(self, x):
 
         if current == self.head:
             self.deleteHead()
-
         elif current == self.tail:
             self.deleteTail()
-
         else:  # Before: 1 <--> 2(current) <--> 3
             current.previous.next = current.next  # 1 --> 3
             current.next.previous = current.previous  # 1 <--> 3
+        self.size -= 1
 
-    def isEmpty(self):  # Will return True if the list is empty
+    def is_empty(self):  # Will return True if the list is empty
         return self.head is None
 
     def display(self):  # Prints contents of the list
         current = self.head
-        while current is not None:
+        while current:
             current.displayLink()
             current = current.next
         print()
 
-
-class Link:
-    next = None  # This points to the link in front of the new link
-    previous = None  # This points to the link behind the new link
-
-    def __init__(self, x):
-        self.value = x
-
-    def displayLink(self):
-        print(f"{self.value}", end=" ")
+    def __len__(self):
+        """
+        >>> linked_list = LinkedList()
+        >>> len(linked_list)
+        0
+        >>> linked_list.insert_head("a")
+        >>> len(linked_list)
+        1
+        >>> linked_list.insert_tail("b")
+        >>> len(linked_list)
+        2
+        >>> _ = linked_list.delete_tail()
+        >>> len(linked_list)
+        1
+        >>> _ = linked_list.delete_head()
+        >>> len(linked_list)
+        0
+        """
+        return self.size