From 46900cb1e5fca2b0b3a96da0fb05c995533b12f7 Mon Sep 17 00:00:00 2001
From: Rian Gallagher <rianogallagher@gmail.com>
Date: Tue, 29 Nov 2016 13:44:09 +0000
Subject: [PATCH] Added Doubly Linked List

---
 .../LinkedList/DoublyLinkedList.py            | 73 +++++++++++++++++++
 1 file changed, 73 insertions(+)
 create mode 100644 data_structures/LinkedList/DoublyLinkedList.py

diff --git a/data_structures/LinkedList/DoublyLinkedList.py b/data_structures/LinkedList/DoublyLinkedList.py
new file mode 100644
index 000000000000..d152e465b399
--- /dev/null
+++ b/data_structures/LinkedList/DoublyLinkedList.py
@@ -0,0 +1,73 @@
+'''
+- 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.
+'''
+class LinkedList:
+    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
+            self.tail = newLink
+        else:
+            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
+        if(self.head == None):
+            self.tail = None
+        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) -->
+    
+    def deleteTail(self):
+        temp = self.tail
+        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
+            current = current.next
+            
+        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
+    
+    def isEmpty(self): #Will return True if the list is empty
+        return(self.head == None)
+        
+    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
+    def __init__(self, x):
+        self.value = x
+    def displayLink(self):
+        print("{}".format(self.value), end=" ")
\ No newline at end of file