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 if(self.head != None): prevNode = self.head tempNode = self.head curNode = self.head.next #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): #3. While the curNode is not null adjust links # (unlink curNode and link it to the reversed list # 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 self.head = prevNode
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 #class Linked List class LinkedList: def __init__(self): self.head = None #Add new element at the end of the list def push_back(self, newElement): newNode = Node(newElement) if(self.head == None): self.head = newNode return else: temp = self.head while(temp.next != None): temp = temp.next temp.next = newNode newNode.prev = temp #reverse the list def reverseList(self): if(self.head != None): prevNode = self.head tempNode = self.head curNode = self.head.next prevNode.next = None prevNode.prev = None while(curNode != None): tempNode = curNode.next curNode.next = prevNode prevNode.prev = curNode prevNode = curNode curNode = tempNode self.head = prevNode #display the content of the list def PrintList(self): temp = self.head 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 MyList = LinkedList() #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