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.
Javascript
<script>// Javascript program to reverse a // doubly linked listvar head;class Node { constructor(val) { this.data = val; this.prev = null; this.next = null; }} /* Function to reverse a Doubly Linked List */function reverse() { var temp = null; var 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 */function push(new_data) { // Allocate node var 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 of 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 */function printList(node) { while (node != null) { document.write(node.data + " "); node = node.next; }}/* Let us create a sorted linked list to test the functions Created linked list will be 10->8->4->2 */push(2);push(4);push(8);push(10);document.write("Original linked list <br/>");printList(head);reverse();document.write("<br/>");document.write("The reversed Linked List is <br/>");printList(head);// This code is contributed by gauravrajput1</script> |
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
Javascript
<script>// Javascript program to reverse a doubly // linked listclass Node { constructor(d) { this.data = d; this.next = this.prev = null; }}let head;// Function to reverse a Doubly // Linked List using Stacksfunction reverse(){ let stack = []; let temp = head; while (temp != null) { stack.push(temp.data); temp = temp.next; } // Added all the elements sequence // 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 function push(new_data){ // Allocate node let 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 of 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 listfunction printList(node){ while (node != null) { document.write(node.data + " "); node = node.next; }}// Driver Code// Let us create a sorted linked list // to test the functions Created linked// list will be 10->8->4->2push(2);push(4);push(8);push(10);document.write("Original linked list <br>");printList(head);reverse();document.write("<br>");document.write("The reversed Linked List is <br>");printList(head);// This code is contributed by rag2127</script> |
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).
Please refer complete article on Reverse a Doubly Linked List for more details!
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