Apex DSA - Reverse a Linked List - Salesforce

 Reverse a Linked List

Problem Statement:
Given a linked list, reverse it and return the new head.

Example:

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Input: 5 -> 4 -> 3 -> 2 -> 1 

Output:  1 -> 2 -> 3 -> 4 -> 5 

Solution :

// Define ListNode as a top-level class

public class ListNode {

    public Integer val;

    public ListNode next;

    public ListNode(Integer val) {

        this.val = val;

        this.next = null;

    }

}

//Build logic for reverse LinkedList

public class ReverseLinkedList {

    public ListNode reverseList(ListNode head) {

        ListNode prev = null;

        ListNode curr = head;

        while (curr != null) {

            ListNode nextTemp = curr.next;

            curr.next = prev;

            prev = curr;

            curr = nextTemp;

        }

        return prev;

    }

}

//Test LinkedList Logic

ListNode head = new ListNode(1);

        head.next = new ListNode(2);

        head.next.next = new ListNode(3);

        head.next.next.next = new ListNode(4);

        head.next.next.next.next = new ListNode(5);

        // Print the original linked list

        system.debug('originalLinkedList : '+head);

        // Reverse the linked list

        ReverseLinkedList op=new ReverseLinkedList();

        ListNode reversedHead = op.reverseList(head);

        // Print the reversed linked list

        system.debug('reversedLinkedList : '+reversedHead);