Reverse a Doubly Linked List | Set 4 (Swapping Data)

Given a Doubly Linked List, we are asked to reverse the list in-place without using any extra space.

Examples:

Input : 1 <--> 2 <--> 5 <--> 6 <--> 7
Output : 7 <--> 6 <--> 5 <--> 2 <--> 1

Input : 11 <--> 22 <--> 33 <--> 22 <--> 1
Output : 1 <--> 22 <--> 33 <--> 22 <--> 11



We have discussed at three methods to reverse a doubly linked list: Reverse a doubly linked list, Reverse a Doubly Linked List (Set 2) and Reverse a Doubly linked list using recursion.

The first two methods work in O(n) time and require no extra space. The first method works by swapping the next and previous pointers of each node. The second method takes each node from the list and adds it to the beginning of the list.

There is another approach that is a bit more intuitive, but also a bit more costly.
This method is similar to reverse an array. To reverse an array, we put two pointers-one at the beginning and another at the end of the list. We then swap the data of the two pointers and advance both pointers toward each other. We stop either when the two pointers meet or when they cross each other. We perform exactly n/2 swaps, and the time complexity is also O(N).

A doubly linked list has both a previous and a next pointer, which means we can traverse in both forward and backward directions in the list. So if we put a pointer( say left pointer), at the beginning of the list and another right pointer at the end of the list, we can move these pointers toward each other by advancing left pointer and receding the right pointer.

Algorithm

Step 1: Set LEFT to head of list
Step 2: Traverse the list and set RIGHT to end of the list 
Step 3: Repeat following steps while LEFT != RIGHT and 
        LEFT->PREV != RIGHT
Step 4: Swap LEFT->DATA and RIGHT->DATA
Step 5: Advance LEFT pointer by one, LEFT = LEFT->NEXT
Step 6: Recede RIGHT pointer by one, i.e RIGHT = RIGHT->PREV
        [END OF LOOP]
Step 7: End

A Note on the comparative efficiency of the three methods

A few things must be mentioned. This method is simple to implement, but it is also more costly when compared to say the pointer-exchange method. This is because we swap data and not pointers. Swapping data can be more costly if the nodes are large complex data types with multiple data members. In contrast, the pointer to the node will always be simpler data type and either of 4 or 8 bytes.

Below is the implementation of the algorithm.

C++

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// Cpp Program to Reverse a List using Data Swapping
  
#include <iostream>
using namespace std;
  
struct Node {
    int data;
    struct Node *prev, *next;
};
  
Node* newNode(int val)
{
    Node* temp = new Node;
    temp->data = val;
    temp->prev = temp->next = nullptr;
    return temp;
}
  
void printList(Node* head)
{
    while (head->next != nullptr) {
        cout << head->data << " <--> ";
        head = head->next;
    }
    cout << head->data << endl;
}
  
// Insert a new node at the head of the list
void insert(Node** head, int val)
{
    Node* temp = newNode(val);
    temp->next = *head;
    (*head)->prev = temp;
    (*head) = temp;
}
  
// Function to reverse the list
void reverseList(Node** head)
{
    Node* left = *head, * right = *head;
  
    // Traverse the list and set right pointer to
    // end of list
    while (right->next != nullptr)
        right = right->next;
  
    // Swap data of left and right pointer and move
    // them towards each other until they meet or 
    // cross each other
    while (left != right && left->prev != right) {
  
        // Swap data of left and right pointer
        swap(left->data, right->data);
  
        // Advance left pointer
        left = left->next;
  
        // Advance right pointer
        right = right->prev;
    }
}
  
// Driver code
int main()
{
    Node* head = newNode(5);
    insert(&head, 4);
    insert(&head, 3);
    insert(&head, 2);
    insert(&head, 1);
  
    printList(head);
    cout << "List After Reversing" << endl;
    reverseList(&head);
    printList(head);
  
    return 0;
}

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Java

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// Java Program to Reverse a List using Data Swapping 
class GFG
{
static class Node
    int data; 
    Node prev, next; 
}; 
  
static Node newNode(int val) 
    Node temp = new Node(); 
    temp.data = val; 
    temp.prev = temp.next = null
    return temp; 
  
static void printList(Node head) 
    while (head.next != null)
    
        System.out.print(head.data+ " <-> "); 
        head = head.next; 
    
    System.out.println( head.data ); 
  
// Insert a new node at the head of the list 
static Node insert(Node head, int val) 
    Node temp = newNode(val); 
    temp.next = head; 
    (head).prev = temp; 
    (head) = temp;
    return head;
  
// Function to reverse the list 
static Node reverseList(Node head) 
    Node left = head, right = head; 
  
    // Traverse the list and set right pointer to 
    // end of list 
    while (right.next != null
        right = right.next; 
  
    // Swap data of left and right pointer and move 
    // them towards each other until they meet or 
    // cross each other 
    while (left != right && left.prev != right)
    
  
        // Swap data of left and right pointer 
        int t = left.data;
        left.data = right.data;
        right.data = t;
  
        // Advance left pointer 
        left = left.next; 
  
        // Advance right pointer 
        right = right.prev; 
    
    return head;
  
// Driver code 
public static void main(String args[])
    Node head = newNode(5); 
    head = insert(head, 4); 
    head = insert(head, 3); 
    head = insert(head, 2); 
    head = insert(head, 1); 
  
    printList(head); 
    System.out.println("List After Reversing"); 
    head=reverseList(head); 
    printList(head); 
  
}
}
  
// This code is contributed by Arnab Kundu 

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C#

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// C# Program to Reverse a List using Data Swapping 
using System;
  
class GFG
{
  
    public class Node
    
        public int data; 
        public Node prev, next; 
    }; 
      
    static Node newNode(int val) 
    
        Node temp = new Node(); 
        temp.data = val; 
        temp.prev = temp.next = null
        return temp; 
    
      
    static void printList(Node head) 
    
        while (head.next != null)
        
            Console.Write(head.data+ " <-> "); 
            head = head.next; 
        
        Console.WriteLine( head.data ); 
    
      
    // Insert a new node at the head of the list 
    static Node insert(Node head, int val) 
    
        Node temp = newNode(val); 
        temp.next = head; 
        (head).prev = temp; 
        (head) = temp;
        return head;
    
      
    // Function to reverse the list 
    static Node reverseList(Node head) 
    
        Node left = head, right = head; 
      
        // Traverse the list and set right pointer to 
        // end of list 
        while (right.next != null
            right = right.next; 
      
        // Swap data of left and right pointer and move 
        // them towards each other until they meet or 
        // cross each other 
        while (left != right && left.prev != right)
        
      
            // Swap data of left and right pointer 
            int t = left.data;
            left.data = right.data;
            right.data = t;
      
            // Advance left pointer 
            left = left.next; 
      
            // Advance right pointer 
            right = right.prev; 
        
        return head;
    
      
    // Driver code 
    public static void Main(String []args)
    
        Node head = newNode(5); 
        head = insert(head, 4); 
        head = insert(head, 3); 
        head = insert(head, 2); 
        head = insert(head, 1); 
      
        printList(head); 
        Console.WriteLine("List After Reversing"); 
        head=reverseList(head); 
        printList(head); 
      
    }
}
  
// This code has been contributed by 29AjayKumar

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Output:

1 <--> 2 <--> 3 <--> 4 <--> 5
List After Reversing
5 <--> 4 <--> 3 <--> 2 <--> 1


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