Reverse a Doubly Linked List
Given a Doubly Linked List, the task is to reverse the given Doubly Linked List.
See below diagrams for example.
(a) Original Doubly Linked List
(b) Reversed Doubly Linked List
Here is a simple method for reversing a Doubly Linked List. All we need to do is swap prev and next pointers for all nodes, change prev of the head (or start) and change the head pointer in the end.
C++
/* C++ program to reverse a doubly linked list */#include <bits/stdc++.h>using namespace std;/* a node of the doubly linked list */class Node{ public: int data; Node *next; Node *prev;};/* Function to reverse a Doubly Linked List */void reverse(Node **head_ref){ Node *temp = NULL; Node *current = *head_ref; /* 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 the head, check for the cases like empty list and list with only one node */ if(temp != NULL ) *head_ref = temp->prev;}/* UTILITY FUNCTIONS *//* Function to insert a node at thebeginging of the Doubly Linked List */void push(Node** head_ref, int new_data){ /* allocate node */ Node* new_node = new Node(); /* put in the data */ new_node->data = new_data; /* since we are adding at the beginning, prev is always NULL */ new_node->prev = NULL; /* link the old list off the new node */ new_node->next = (*head_ref); /* change prev of head node to new node */ if((*head_ref) != NULL) (*head_ref)->prev = new_node ; /* move the head to point to the new node */ (*head_ref) = new_node;}/* Function to print nodes in a given doubly linked listThis function is same as printList() of singly linked list */void printList(Node *node){ while(node != NULL) { cout << node->data << " "; node = node->next; }}/* Driver code */int main(){ /* Start with the empty list */ Node* head = NULL; /* Let us create a sorted linked list to test the functions Created linked list will be 10->8->4->2 */ push(&head;, 2); push(&head;, 4); push(&head;, 8); push(&head;, 10); cout << "Original Linked list" << endl; printList(head); /* Reverse doubly linked list */ reverse(&head;); cout << "\nReversed Linked list" << endl; printList(head); return 0;}// This code is contributed by rathbhupendra |
C
/* Program to reverse a doubly linked list */#include <stdio.h>#include <stdlib.h>/* a node of the doubly linked list */struct Node{ int data; struct Node *next; struct Node *prev; };/* Function to reverse a Doubly Linked List */void reverse(struct Node **head_ref){ struct Node *temp = NULL; struct Node *current = *head_ref; /* 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_ref = temp->prev;} /* UTILITY FUNCTIONS *//* Function to insert a node at the beginging of the Doubly Linked List */void push(struct Node** head_ref, int new_data){ /* allocate node */ struct Node* new_node = (struct Node*) malloc(sizeof(struct Node)); /* put in the data */ new_node->data = new_data; /* since we are adding at the beginning, prev is always NULL */ new_node->prev = NULL; /* link the old list off the new node */ new_node->next = (*head_ref); /* change prev of head node to new node */ if((*head_ref) != NULL) (*head_ref)->prev = new_node ; /* move the head to point to the new node */ (*head_ref) = new_node;}/* Function to print nodes in a given doubly linked list This function is same as printList() of singly linked list */void printList(struct Node *node){ while(node!=NULL) { printf("%d ", node->data); node = node->next; }}/* Driver code*/int main(){ /* Start with the empty list */ struct Node* head = NULL; /* Let us create a sorted linked list to test the functions Created linked list will be 10->8->4->2 */ push(&head;, 2); push(&head;, 4); push(&head;, 8); push(&head;, 10); printf("\n Original Linked list "); printList(head); /* Reverse doubly linked list */ reverse(&head;); printf("\n Reversed Linked list "); printList(head); getchar();} |
Java
// Java program to reverse a doubly linked listclass LinkedList { static Node head; static class Node { int data; Node next, prev; Node(int d) { data = d; next = prev = null; } } /* Function to reverse a Doubly Linked List */ void reverse() { 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; } } /* UTILITY FUNCTIONS */ /* Function to insert a node at the beginning of the * Doubly Linked List */ void push(int new_data) { /* allocate node */ Node new_node = new Node(new_data); /* since we are adding at the beginning, prev is always NULL */ new_node.prev = null; /* link the old list off the new node */ new_node.next = head; /* change prev of head node to new node */ if (head != null) { head.prev = new_node; } /* move the head to point to the new node */ head = new_node; } /* Function to print nodes in a given doubly linked list This function is same as printList() of singly linked list */ void printList(Node node) { while (node != null) { System.out.print(node.data + " "); node = node.next; } } public static void main(String[] args) { LinkedList list = new LinkedList(); /* Let us create a sorted linked list to test the functions Created linked list will be 10->8->4->2 */ list.push(2); list.push(4); list.push(8); list.push(10); System.out.println("Original linked list "); list.printList(head); list.reverse(); System.out.println(""); System.out.println("The reversed Linked List is "); list.printList(head); }}// This code has been contributed by Mayank Jaiswal |
Python
# Program to reverse a doubly linked list# A node of the doubly linked listclass Node: # Constructor to create a new node def __init__(self, data): self.data = data self.next = None self.prev = Noneclass DoublyLinkedList: # Constructor for empty Doubly Linked List def __init__(self): self.head = None # Function reverse a Doubly Linked List def reverse(self): temp = None current = self.head # Swap next and prev for all nodes of # doubly linked list while current is not None: 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 is not None: self.head = temp.prev # Given a reference to the head of a list and an # integer,inserts a new node on the front of list def push(self, new_data): # 1. Allocates node # 2. Put the data in it new_node = Node(new_data) # 3. Make next of new node as head and # previous as None (already None) new_node.next = self.head # 4. change prev of head node to new_node if self.head is not None: self.head.prev = new_node # 5. move the head to point to the new node self.head = new_node def printList(self, node): while(node is not None): print node.data, node = node.next# Driver codedll = DoublyLinkedList()dll.push(2)dll.push(4)dll.push(8)dll.push(10)print "\nOriginal Linked List"dll.printList(dll.head)# Reverse doubly linked listdll.reverse()print "\n Reversed Linked List"dll.printList(dll.head)# This code is contributed by Nikhil Kumar Singh(nickzuck_007) |
C#
// A C# program to reverse a doubly linked listusing System;public class LinkedList { static Node head; class Node { public int data; public Node next, prev; public Node(int d) { data = d; next = prev = null; } } /* Function to reverse a Doubly Linked List */ void reverse() { 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; } } /* UTILITY FUNCTIONS */ /* Function to insert a node at the beginging of the Doubly Linked List */ void push(int new_data) { /* allocate node */ Node new_node = new Node(new_data); /* since we are adding at the beginning, prev is always NULL */ new_node.prev = null; /* link the old list off the new node */ new_node.next = head; /* change prev of head node to new node */ if (head != null) { head.prev = new_node; } /* move the head to point to the new node */ head = new_node; } /* Function to print nodes in a given doubly linked list This function is same as printList() of singly linked lsit */ void printList(Node node) { while (node != null) { Console.Write(node.data + " "); node = node.next; } } // Driver code public static void Main(String[] args) { LinkedList list = new LinkedList(); /* Let us create a sorted linked list to test the functions Created linked list will be 10->8->4->2 */ list.push(2); list.push(4); list.push(8); list.push(10); Console.WriteLine("Original linked list "); list.printList(head); list.reverse(); Console.WriteLine(""); Console.WriteLine("The reversed Linked List is "); list.printList(head); }}// This code is contributed by 29AjayKumar |
Output:
Original linked list 10 8 4 2 The reversed Linked List is 2 4 8 10
Time Complexity: O(N), where N denotes the number of nodes in the doubly linked list.
Auxiliary Space: O(1)
We can also swap data instead of pointers to reverse the Doubly Linked List. Method used for reversing array can be used to swap data. Swapping data can be costly compared to pointers if the size of the data item(s) is more.
Please write comments if you find any of the above codes/algorithms incorrect, or find better ways to solve the same problem.
Method 2:
The same question can also be done by using Stacks.
Steps:
- Keep pushing the node’s data in the stack. -> O(n)
- The keep popping the elements out and updating the Doubly Linked List
C++
// C++ program to reverse a doubly linked list#include <bits/stdc++.h>using namespace std;struct LinkedList { struct Node { int data; Node *next, *prev; Node(int d) { data = d; next = prev = NULL; } }; Node* head = NULL; /* Function to reverse a Doubly Linked List using Stacks */ void reverse() { stack<int> st; Node* temp = head; while (temp != NULL) { st.push(temp->data); temp = temp->next; } // added all the elements sequemce wise in the // st temp = head; while (temp != NULL) { temp->data = st.top(); st.pop(); temp = temp->next; } // popped all the elements and the added in the // linked list, // which are in the reversed order-> } /* UTILITY FUNCTIONS */ /* Function to insert a node at the beginning of the * Doubly Linked List */ void Push(int new_data) { /* allocate node */ Node* new_node = new Node(new_data); /* since we are adding at the beginning, prev is always NULL */ new_node->prev = NULL; /* link the old list off the new node */ new_node->next = head; /* change prev of head node to new node */ if (head != NULL) { head->prev = new_node; } /* move the head to point to the new node */ head = new_node; } /* Function to print nodes in a given doubly linked list This function is same as printList() of singly linked list */ void printList(Node* node) { while (node) { cout << node->data << " "; node = node->next; } }};// Driver Codeint main(){ LinkedList list; /* Let us create a sorted linked list to test the functions Created linked list will be 10->8->4->2 */ list.Push(2); list.Push(4); list.Push(8); list.Push(10); cout << "Original linked list " << endl; list.printList(list.head); list.reverse(); cout << endl; cout << "The reversed Linked List is " << endl; list.printList(list.head);}// This code is contributed by Pratham76 |
Java
// Java program to reverse a doubly linked listimport java.util.*;class LinkedList { static Node head; static class Node { int data; Node next, prev; Node(int d) { data = d; next = prev = null; } } /* Function to reverse a Doubly Linked List using Stacks */ void reverse() { Stack<Integer> stack = new Stack<>(); Node temp = head; while (temp != null) { stack.push(temp.data); temp = temp.next; } // added all the elements sequemce wise in the // stack temp = head; while (temp != null) { temp.data = stack.pop(); temp = temp.next; } // popped all the elements and the added in the // linked list, // which are in the reversed order. } /* UTILITY FUNCTIONS */ /* Function to insert a node at the beginning of the * Doubly Linked List */ void push(int new_data) { /* allocate node */ Node new_node = new Node(new_data); /* since we are adding at the beginning, prev is always NULL */ new_node.prev = null; /* link the old list off the new node */ new_node.next = head; /* change prev of head node to new node */ if (head != null) { head.prev = new_node; } /* move the head to point to the new node */ head = new_node; } /* Function to print nodes in a given doubly linked list This function is same as printList() of singly linked list */ void printList(Node node) { while (node != null) { System.out.print(node.data + " "); node = node.next; } } // Driver Code public static void main(String[] args) { LinkedList list = new LinkedList(); /* Let us create a sorted linked list to test the functions Created linked list will be 10->8->4->2 */ list.push(2); list.push(4); list.push(8); list.push(10); System.out.println("Original linked list "); list.printList(head); list.reverse(); System.out.println(""); System.out.println("The reversed Linked List is "); list.printList(head); }}// This code has been contributed by Rashita Mehta |
Python3
"""function to reverse a doubly-linked listswap next and prev pointers for all the nodeschange prev of the head nodechange head pointer"""class Node: def __init__(self, data): self.data = data self.next = None self.prev = Noneclass DoublyLinkedList: def __init__(self): self.head = None """ method to reverse a Doubly-Linked List using Stacks """ def reverseUsingStacks(self): stack = [] temp = self.head while temp is not None: stack.append(temp.data) temp = temp.next # Add all the elements in the stack # in a sequence to the stack temp = self.head while temp is not None: temp.data = stack.pop() temp = temp.next # Popped all the elements and the # added in the linked list, # in a reversed order. """ method to push a new item before the head """ def push(self, new_data): new_node = Node(new_data) new_node.next = self.head if self.head is not None: self.head.prev = new_node self.head = new_node """ method to traverse the doubly-linked list and print every node in the list """ def printList(self, node): while(node is not None): print(node.data) node = node. next# driver program for the doubly-linked listdll = DoublyLinkedList()dll.push(2)dll.push(4)dll.push(8)dll.push(10)print("original doubly-linked list")dll.printList(dll.head)# reverse a doubly-linked listdll.reverseUsingStacks()print(" reversed doubly-linked list")dll.printList(dll.head) |
C#
// C# program to reverse a doubly linked listusing System;using System.Collections;using System.Collections.Generic;class LinkedList { public static Node head; public class Node { public int data; public Node next, prev; public Node(int d) { data = d; next = prev = null; } } /* Function to reverse a Doubly Linked List using Stacks */ public void reverse() { Stack stack = new Stack(); Node temp = head; while (temp != null) { stack.Push(temp.data); temp = temp.next; } // added all the elements sequemce wise in the // stack temp = head; while (temp != null) { temp.data = (int)stack.Pop(); temp = temp.next; } // popped all the elements and the added in the // linked list, // which are in the reversed order. } /* UTILITY FUNCTIONS */ /* Function to insert a node at the beginning of the * Doubly Linked List */ public void Push(int new_data) { /* allocate node */ Node new_node = new Node(new_data); /* since we are adding at the beginning, prev is always NULL */ new_node.prev = null; /* link the old list off the new node */ new_node.next = head; /* change prev of head node to new node */ if (head != null) { head.prev = new_node; } /* move the head to point to the new node */ head = new_node; } /* Function to print nodes in a given doubly linked list This function is same as printList() of singly linked list */ public void printList(Node node) { while (node != null) { Console.Write(node.data + " "); node = node.next; } } // Driver Code public static void Main(string[] args) { LinkedList list = new LinkedList(); /* Let us create a sorted linked list to test the functions Created linked list will be 10->8->4->2 */ list.Push(2); list.Push(4); list.Push(8); list.Push(10); Console.WriteLine("Original linked list "); list.printList(head); list.reverse(); Console.WriteLine(""); Console.WriteLine("The reversed Linked List is "); list.printList(head); }}// This code is contributed by rutvik_56 |
Output
Original linked list 10 8 4 2 The reversed Linked List is 2 4 8 10
Time Complexity: O(N)
Auxiliary Space: O(N)
In this method, we traverse the linked list once and add elements to the stack, and again traverse the whole for updating all the elements. The whole takes 2n time, which is the time complexity of O(n).
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