# Reverse a Doubly Linked List

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.

 `/* Program to reverse a doubly linked list */` `#include ` `#include ` `/* 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``();` `}`

Time Complexity: O(n)

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