MergeTwoSortedLinkedLists.java

/*
You are given two sorted linked lists. Your task is to merge these two lists into a single sorted linked list. The result should also be a sorted list. 
You can assume that both input lists are already sorted in ascending order.

Write a function mergeLists(head1, head2) that merges these two linked lists.
    Input: Two linked lists represented by their head nodes.
    Output: A reference to the head of the new merged and sorted linked list.

Example:
    List 1: 1 → 3 → 5
    List 2: 2 → 4 → 6

Output: 1 → 2 → 3 → 4 → 5 → 6
*/

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 MergeTwoSortedLinkedLists {

    static class SinglyLinkedListNode {
        public int data;
        public SinglyLinkedListNode next;

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

    static class SinglyLinkedList {
        public SinglyLinkedListNode head;
        public SinglyLinkedListNode tail;

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

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

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

            this.tail = node;
        }
    }

    public static void printSinglyLinkedList(SinglyLinkedListNode 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 mergeLists function below.

    /*
     * For your reference:
     *
     * SinglyLinkedListNode {
     *     int data;
     *     SinglyLinkedListNode next;
     * }
     *
     */
    static SinglyLinkedListNode mergeLists(SinglyLinkedListNode head1, SinglyLinkedListNode head2) 
    {
        SinglyLinkedListNode placeHolderNode = new SinglyLinkedListNode(0);
        SinglyLinkedListNode endOfLinkedListNode = placeHolderNode;
        
        while(head1 != null && head2 != null)
        {
            if(head1.data <= head2.data)
            {
                endOfLinkedListNode.next = head1;
                head1 = head1.next;
            }
            
            else
            {
                endOfLinkedListNode.next = head2;
                head2 = head2.next;
            }
            
            endOfLinkedListNode = endOfLinkedListNode.next;
        }
        
        if(head1 != null)
        {
            endOfLinkedListNode.next = head1;
        }
        
        else
        {
            if(head2 != null)
            {
                endOfLinkedListNode.next = head2;
            }
        }
        
        return placeHolderNode.next;
    }
    
    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 tests = scanner.nextInt();
        scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");

        for (int testsItr = 0; testsItr < tests; testsItr++) {
            SinglyLinkedList llist1 = new SinglyLinkedList();

            int llist1Count = scanner.nextInt();
            scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");

            for (int i = 0; i < llist1Count; i++) {
                int llist1Item = scanner.nextInt();
                scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");

                llist1.insertNode(llist1Item);
            }
          
          	SinglyLinkedList llist2 = new SinglyLinkedList();

            int llist2Count = scanner.nextInt();
            scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");

            for (int i = 0; i < llist2Count; i++) {
                int llist2Item = scanner.nextInt();
                scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");

                llist2.insertNode(llist2Item);
            }

            SinglyLinkedListNode llist3 = mergeLists(llist1.head, llist2.head);

            printSinglyLinkedList(llist3, " ", bufferedWriter);
            bufferedWriter.newLine();
        }

        bufferedWriter.close();

        scanner.close();
    }
}