Write a C function to reverse a 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

/* 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 begining, 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 lsit */ void printList(struct Node *node) { while(node!=NULL) { printf("%d ", node->data); node = node->next; } } /* Drier program to test above functions*/ 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 list 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 */ 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 begining, 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) { 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 doublly linked list class Node: # Constructor to create a new node def __init__(self, data): self.data = data self.next = None self.prev = None class 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 program to test the above functions dll = DoublyLinkedList() dll.push(2); dll.push(4); dll.push(8); dll.push(10); print "\nOriginal Linked List" dll.printList(dll.head) # Reverse doubly linked list dll.reverse() print "\n Reversed Linked List" dll.printList(dll.head) # This code is contributed by Nikhil Kumar Singh(nickzuck_007)

Time Complexity: O(n)

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 size of 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.

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