Update doubly_linked_list.py (TheAlgorithms#545)

Author: adityasharma1234

data_structures/linked_list/doubly_linked_list.py

@@ -2,49 +2,50 @@
 - A linked list is similar to an array, it holds values. However, links in a linked list do not have indexes.
 - This is an example of a double ended, doubly linked list.
 - 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 efficent'''
 from __future__ import print_function
 
 
-class LinkedList:
+class LinkedList:           #making main class named linked list
     def __init__(self):
         self.head = None
         self.tail = None
 
     def insertHead(self, x):
-        newLink = Link(x) #Create a new link with a value attached to it
-        if(self.isEmpty() == True): #Set the first element added to be the tail
+        newLink = Link(x)                            #Create a new link with a value attached to it
+        if(self.isEmpty() == True):                  #Set the first element added to be the tail
             self.tail = newLink
         else:
-            self.head.previous = newLink # newLink <-- currenthead(head)
-        newLink.next = self.head # newLink <--> currenthead(head)
-        self.head = newLink # newLink(head) <--> oldhead
+            self.head.previous = newLink             # newLink <-- currenthead(head)
+        newLink.next = self.head                     # newLink <--> currenthead(head)
+        self.head = newLink                          # newLink(head) <--> oldhead
 
     def deleteHead(self):
         temp = self.head
-        self.head = self.head.next # oldHead <--> 2ndElement(head) 
-        self.head.previous = None # oldHead --> 2ndElement(head) nothing pointing at it so the old head will be removed
+        self.head = self.head.next                   # oldHead <--> 2ndElement(head) 
+        self.head.previous = None                    # oldHead --> 2ndElement(head) nothing pointing at it so the old head will be removed
         if(self.head is None):
-            self.tail = None
+            self.tail = None                         #if empty linked list
         return temp
 
     def insertTail(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) -->
+        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) -->
 
     def deleteTail(self):
         temp = self.tail
-        self.tail = self.tail.previous # 2ndLast(tail) <--> oldTail --> None
-        self.tail.next = None # 2ndlast(tail) --> None
+        self.tail = self.tail.previous              # 2ndLast(tail) <--> oldTail --> None
+        self.tail.next = None                       # 2ndlast(tail) --> None
         return temp
 
     def delete(self, x):
         current = self.head
 
-        while(current.value != x): # Find the position to delete
+        while(current.value != x):                  # Find the position to delete
             current = current.next
 
         if(current == self.head):
@@ -57,19 +58,19 @@ def delete(self, x):
             current.previous.next = current.next # 1 --> 3
             current.next.previous = current.previous # 1 <--> 3
 
-    def isEmpty(self): #Will return True if the list is empty
+    def isEmpty(self):                               #Will return True if the list is empty
         return(self.head is None)
 
-    def display(self): #Prints contents of the list
+    def display(self):                                #Prints contents of the list
         current = self.head
         while(current != None):
             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
+    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):