In this post we’ll see a Java program to reverse a doubly linked list.
Reversing a doubly linked list
In a doubly Linked list each node stores reference to both next and previous nodes. For reversing a doubly linked list, for each node previous and next references should be swapped.
Once the doubly linked list is reversed head and tail references should also be adjusted accordingly.
Java program to reverse a doubly linked list can be written using both-
- Iterative approach
- Recursive approach
Java program to reverse a doubly linked list – Iterative
public class ReverseDLL {
private Node head;
private Node tail;
static class Node{
//data
int i;
// next node in the list
Node next;
// previous node in the list
Node prev;
Node(int i){
this.i = i;
}
public void displayData(){
System.out.print(i + "");
}
}
// constructor
public ReverseDLL(){
this.head = null;
this.tail = null;
}
public boolean isEmpty(){
return head == null;
}
public void insertFirst(int i){
//Create a new node
Node newNode = new Node(i);
// if first insertion tail should
// also point to this node
if(isEmpty()){
tail = newNode;
}else{
head.prev = newNode;
}
newNode.next = head;
head = newNode;
}
// Method for forward traversal
public void displayForward(){
System.out.println("Forward display");
Node current = head;
while(current != null){
current.displayData();
current = current.next;
}
System.out.println("");
}
// Method to traverse and display all nodes
public void displayBackward(){
System.out.println("Backward display");
Node current = tail;
while(current != null){
current.displayData();
current = current.prev;
}
System.out.println("");
}
// Method for reversing doubly linked list
public void reverseList(){
Node previous = null;
//change reference head becomes tail in reversal
tail = head;
Node current = head;
while(current != null){
// swap prev and next for the current node
previous = current.prev;
current.prev = current.next;
current.next = previous;
// to move to next node current.prev has to be called
// as that has reference to next node now
current = current.prev;
}
if(previous != null){
head = previous.prev;
}
}
public static void main(String[] args) {
ReverseDLL list = new ReverseDLL();
list.insertFirst(4);
list.insertFirst(3);
list.insertFirst(2);
list.insertFirst(1);
list.displayForward();
list.reverseList();
list.displayForward();
list.displayBackward();
}
}
Output
Forward display
1 2 3 4
Forward display
4 3 2 1
Backward display
1 2 3 4
Java program to reverse a doubly linked list – Recursive
In the recursive method for reversing a doubly linked linked list method is called passing the first node then the method is recursively called by passing the next node (node.next).
public class ReverseDLL {
private Node head;
private Node tail;
static class Node{
//data
int i;
// next node in the list
Node next;
// previous node in the list
Node prev;
Node(int i){
this.i = i;
}
public void displayData(){
System.out.print(i + "");
}
}
// constructor
public ReverseDLL(){
this.head = null;
this.tail = null;
}
public boolean isEmpty(){
return head == null;
}
public void insertFirst(int i){
//Create a new node
Node newNode = new Node(i);
// if first insertion tail should
// also point to this node
if(isEmpty()){
tail = newNode;
}else{
head.prev = newNode;
}
newNode.next = head;
head = newNode;
}
// Method for forward traversal
public void displayForward(){
System.out.println("Forward display");
Node current = head;
while(current != null){
current.displayData();
current = current.next;
}
System.out.println("");
}
// Method to traverse and display all nodes
public void displayBackward(){
System.out.println("Backward display");
Node current = tail;
while(current != null){
current.displayData();
current = current.prev;
}
System.out.println("");
}
public Node reverseListRec(Node current){
if (current == null) {
return current;
}
if (current.next == null) {
current.prev = null;
return current;
}
Node node = reverseListRec(current.next);
current.next.next = current;
current.prev = current.next;
current.next = null;
return node;
}
public static void main(String[] args) {
ReverseDLL list = new ReverseDLL();
list.insertFirst(4);
list.insertFirst(3);
list.insertFirst(2);
list.insertFirst(1);
list.displayForward();
// change tail reference before calling reverse method
list.tail = list.head;
Node node = list.reverseListRec(list.head);
// change head to point to returned node
list.head = node;
//list.reverseList();
list.displayForward();
list.displayBackward();
}
}
Output
Forward display
1 2 3 4
Forward display
4 3 2 1
Backward display
1 2 3 4
In the recursive method for reversing doubly linked list reference is reversed in the following lines.
Following image tries to clarify what this line does.
current.next.next = current;
current.prev = current.next;
That's all for this topic How to Reverse a Doubly Linked List in Java. If you have any doubt or any suggestions to make please drop a comment. Thanks!
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