Java Program for Reverse a linked list

Last Updated : 17 Jun, 2022
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Given a pointer to the head node of a linked list, the task is to reverse the linked list. We need to reverse the list by changing links between nodes. Examples:

Input: Head of following linked list  
       1->2->3->4->NULL
Output: Linked list should be changed to,
       4->3->2->1->NULL

Input: Head of following linked list  
       1->2->3->4->5->NULL
Output: Linked list should be changed to,
       5->4->3->2->1->NULL

Input: NULL
Output: NULL

Input: 1->NULL
Output: 1->NULL

Iterative Method:

Java 
// Java program for reversing the linked list

class LinkedList {

    static Node head;

    static class Node {

        int data;
        Node next;

        Node(int d) {
            data = d;
            next = null;
        }
    }

    /* Function to reverse the linked list */
    Node reverse(Node node) {
        Node prev = null;
        Node current = node;
        Node next = null;
        while (current != null) {
            next = current.next;
            current.next = prev;
            prev = current;
            current = next;
        }
        node = prev;
        return node;
    }

    // prints content of double 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();
        list.head = new Node(85);
        list.head.next = new Node(15);
        list.head.next.next = new Node(4);
        list.head.next.next.next = new Node(20);
        
        System.out.println("Given Linked list");
        list.printList(head);
        head = list.reverse(head);
        System.out.println("");
        System.out.println("Reversed linked list ");
        list.printList(head);
    }
}


// This code has been contributed by Mayank Jaiswal

Output
Given Linked list
85 15 4 20 
Reversed linked list 
20 4 15 85

Time Complexity: O(N), where N is the length of the given linked list
Auxiliary Space: O(1)

A Simpler and Tail Recursive Method:

Java 
// Java program for reversing the Linked list

class LinkedList {

    static Node head;

    static class Node {

        int data;
        Node next;

        Node(int d) {
            data = d;
            next = null;
        }
    }

    // A simple and tail recursive function to reverse
    // a linked list. prev is passed as NULL initially.
    Node reverseUtil(Node curr, Node prev) {

        /* If last node mark it head*/
        if (curr.next == null) {
            head = curr;

            /* Update next to prev node */
            curr.next = prev;
            return null;
        }

        /* Save curr->next node for recursive call */
        Node next1 = curr.next;

        /* and update next ..*/
        curr.next = prev;

        reverseUtil(next1, curr);
        return head;
    }

    // prints content of double 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();
        list.head = new Node(1);
        list.head.next = new Node(2);
        list.head.next.next = new Node(3);
        list.head.next.next.next = new Node(4);
        list.head.next.next.next.next = new Node(5);
        list.head.next.next.next.next.next = new Node(6);
        list.head.next.next.next.next.next.next = new Node(7);
        list.head.next.next.next.next.next.next.next = new Node(8);

        System.out.println("Original Linked list ");
        list.printList(head);
        Node res = list.reverseUtil(head, null);
        System.out.println("");
        System.out.println("");
        System.out.println("Reversed linked list ");
        list.printList(res);
    }
}
// This code has been contributed by Mayank Jaiswal

Output
Original Linked list 
1 2 3 4 5 6 7 8 

Reversed linked list 
8 7 6 5 4 3 2 1

Time Complexity: O(N), where N is the length of the given linked list
Auxiliary Space: O(1)

Please refer complete article on Reverse a linked list for more details!


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