Reverse a doubly linked list


Problem Statement :


This challenge is part of a tutorial track by MyCodeSchool

Given the pointer to the head node of a doubly linked list, reverse the order of the nodes in place. That is, change the next and prev pointers of the nodes so that the direction of the list is reversed. Return a reference to the head node of the reversed list.

Note: The head node might be NULL to indicate that the list is empty.

Function Description

Complete the reverse function in the editor below.

reverse has the following parameter(s):

DoublyLinkedListNode head: a reference to the head of a DoublyLinkedList
Returns
- DoublyLinkedListNode: a reference to the head of the reversed list

Input Format

The first line contains an integer t, the number of test cases.

Each test case is of the following format:

The first line contains an integer n, the number of elements in the linked list.
The next n lines contain an integer each denoting an element of the linked list.


Solution :



title-img


                            Solution in C :

In C :



// Complete the reverse function below.

/*
 * For your reference:
 *
 * DoublyLinkedListNode {
 *     int data;
 *     DoublyLinkedListNode* next;
 *     DoublyLinkedListNode* prev;
 * };
 *
 */
DoublyLinkedListNode* reverse(DoublyLinkedListNode* head) {
struct DoublyLinkedListNode *prev,*curr,*next;
    curr=head;
    prev=NULL;
    while(curr)
    {
        next=curr->next;
        curr->next=prev;
        curr->prev=next;
        if(next==NULL)break;
        prev=curr;curr=next;
    }
    curr->prev=NULL;return curr;

}
                        

                        Solution in C++ :

In C++ :



/*
   Reverse a doubly linked list, input list may also be empty
   Node is defined as
   struct Node
   {
     int data;
     Node *next;
     Node *prev
   }
*/
Node* Reverse(Node* head)
{
    // Complete this function
    // Do not write the main method. 
    
    Node *temp = NULL;  
     Node *current = head;
      
     /* swap next and prev for all nodes of 
       doubly linked list */
     while (current !=  NULL)
     {
       temp = current->prev;
       current->prev = current->next;
       current->next = temp;              
       current = current->prev;
     }      
      
     /* Before changing head, check for the cases like empty 
        list and list with only one node */
     if(temp != NULL )
        head = temp->prev;
    
    return head;
}
                    

                        Solution in Java :

In Java :




/*
  Insert Node at the end of a linked list 
  head pointer input could be NULL as well for empty list
  Node is defined as 
  class Node {
     int data;
     Node next;
     Node prev;
  }
*/

Node Reverse(Node head) {
    
    if(head==null)
        return null;
    
    if(head.next==null)
        return head;
    
    Node temp=head;
    Node next=temp.next;
    while(next!=null)
        {
          
           temp.next=temp.prev;
        temp.prev=next;
        temp=next;
        next=next.next;
        
    }
    
    temp.next=temp.prev;
    temp.prev=null;
    return temp;

}
                    

                        Solution in Python : 
                            
In python3 :



def Reverse(head):
	if head == None or head.next == None:
		return head

	prev = head
	curr = head.next

	while curr.next != None:
		prev, curr = curr, curr.next

	head = curr

	while prev != None:
		curr.prev = curr.next
		curr.next = prev
		prev.next = prev.prev
		prev.prev = curr
		prev, curr = prev.next, prev

	return head
                    

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