Let's Code_: HackerRank: Reverse a doubly linked list

Sunday, February 14, 2021

HackerRank: Reverse a doubly linked list

Problem Description: https://www.hackerrank.com/challenges/reverse-a-doubly-linked-list/problem

Runtime complexity of my below code: O(n) where n is the size of the linked list.

Solution approach: Recursive

My Java Solution:

import java.io.*;
import java.math.*;
import java.security.*;
import java.text.*;
import java.util.*;
import java.util.concurrent.*;
import java.util.regex.*;

public class Solution {

    static class DoublyLinkedListNode {
        public int data;
        public DoublyLinkedListNode next;
        public DoublyLinkedListNode prev;

        public DoublyLinkedListNode(int nodeData) {
            this.data = nodeData;
            this.next = null;
            this.prev = null;
        }
    }

    static class DoublyLinkedList {
        public DoublyLinkedListNode head;
        public DoublyLinkedListNode tail;

        public DoublyLinkedList() {
            this.head = null;
            this.tail = null;
        }

        public void insertNode(int nodeData) {
            DoublyLinkedListNode node = new DoublyLinkedListNode(nodeData);

            if (this.head == null) {
                this.head = node;
            } else {
                this.tail.next = node;
                node.prev = this.tail;
            }

            this.tail = node;
        }
    }

    public static void printDoublyLinkedList(DoublyLinkedListNode node, String sep, BufferedWriter bufferedWriter) throws IOException {
        while (node != null) {
            bufferedWriter.write(String.valueOf(node.data));

            node = node.next;

            if (node != null) {
                bufferedWriter.write(sep);
            }
        }
    }

    // Complete the reverse function below.

    /*
     * For your reference:
     *
     * DoublyLinkedListNode {
     *     int data;
     *     DoublyLinkedListNode next;
     *     DoublyLinkedListNode prev;
     * }
     *
     */
    static DoublyLinkedListNode reverse(DoublyLinkedListNode head) {
        return reverseRecur(head, head.next);
    }
    
    
    private static DoublyLinkedListNode reverseRecur(DoublyLinkedListNode current, DoublyLinkedListNode nextNode) { // 2, 3    3, 4   4, \0
        if (current == null)
            return current;

        if (nextNode == null && current.prev == null) {
            return current;
        }

        //Node nextNode = current.next; 2
        DoublyLinkedListNode prevNode = current.prev; // \0  1 2 3

        if (nextNode == null) {
            current.prev = null;
            return current;  // 4
        }

        //Assume reversed till current.
        DoublyLinkedListNode nextToNext = nextNode.next; // 4  \0

        // 1 <-> 2 <-> 3 <-> 4  ->   4<->3<->2<->1
        nextNode.next = current;  //4 <-> 3 <-> 2 <-> 1 -> \0
        current.prev = nextNode;
        current.next = prevNode;

        return reverseRecur(nextNode, nextToNext); // 3, 4    4, \0

    }


    private static final Scanner scanner = new Scanner(System.in);

    public static void main(String[] args) throws IOException {
        BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(System.getenv("OUTPUT_PATH")));

        int t = scanner.nextInt();
        scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");

        for (int tItr = 0; tItr < t; tItr++) {
            DoublyLinkedList llist = new DoublyLinkedList();

            int llistCount = scanner.nextInt();
            scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");

            for (int i = 0; i < llistCount; i++) {
                int llistItem = scanner.nextInt();
                scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");

                llist.insertNode(llistItem);
            }

            DoublyLinkedListNode llist1 = reverse(llist.head);

            printDoublyLinkedList(llist1, " ", bufferedWriter);
            bufferedWriter.newLine();
        }

        bufferedWriter.close();

        scanner.close();
    }
}