# Python - Reverse the Doubly Linked List

While working with a Doubly Linked List, sometimes it is required to reverse it. Reversing a List produces following result: if the given List is 10->20->30->40->50, after reversing the List the List becomes 50->40->30->20->10.

Reversing a List requires creating three nodes, considering that the list is not empty, which are as follows: tempNode pointing to head, prevNode pointing to head and curNode pointing to next of head. Then make next and previous of the prevNode as null to make the first node as last node of the reversed list. After that, while the curNode is not null adjust links (unlink curNode and link it to the reversed list from front and modify curNode and prevNode to the next element in the list). At last, make the prevNode (last node) as head.

The function reverseList is created for this purpose. It is a 4-step process.

```def reverseList(self):
#1. If head is not null create three nodes
#   prevNode - pointing to head,
#   tempNode - pointing to head,
#   curNode - pointing to next of head

#2. assign next and previous of the
#   prevNode as null to make the first
#   node as last node of the reversed list
prevNode.next = None
prevNode.prev = None

while(curNode != None):
#   from front and modify curNode and prevNode)
tempNode = curNode.next
curNode.next = prevNode
prevNode.prev = curNode
prevNode = curNode
curNode = tempNode

#4. Make prevNode (last node) as head
```

The below is a complete program that uses above discussed concept to reverse a given doubly linked list.

```# node structure
class Node:
def __init__(self, data):
self.data = data
self.next = None
self.prev = None

def __init__(self):

#Add new element at the end of the list
def push_back(self, newElement):
newNode = Node(newElement)
return
else:
while(temp.next != None):
temp = temp.next
temp.next = newNode
newNode.prev = temp

#reverse the list
def reverseList(self):

prevNode.next = None
prevNode.prev = None

while(curNode != None):
tempNode = curNode.next
curNode.next = prevNode
prevNode.prev = curNode
prevNode = curNode
curNode = tempNode

#display the content of the list
def PrintList(self):
if(temp != None):
print("The list contains:", end=" ")
while (temp != None):
print(temp.data, end=" ")
temp = temp.next
print()
else:
print("The list is empty.")

# test the code

#Add five elements in the list.
MyList.push_back(10)
MyList.push_back(20)
MyList.push_back(30)
MyList.push_back(40)
MyList.push_back(50)

#Display the content of the list.
MyList.PrintList()

#Reversing the list.
MyList.reverseList()

#Display the content of the list.
MyList.PrintList()
```

The above code will give the following output:

```The list contains: 10 20 30 40 50
The list contains: 50 40 30 20 10
```

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