Java Program To Merge K Sorted Linked Lists Using Min Heap - Set 2

// Java implementation to merge
// k sorted linked lists
// Using MIN HEAP method
import java.util.PriorityQueue;
import java.util.Comparator;
public class MergeKLists 
{
    // Function to merge k sorted 
    // linked lists
    public static Node mergeKSortedLists(
                  Node arr[], int k)
    {
        Node head = null, last = null;

        // priority_queue 'pq' implemented
        // as min heap with the
        // help of 'compare' function
        PriorityQueue<Node> pq = 
                new PriorityQueue<>(new Comparator<Node>() 
        {
            public int compare(Node a, Node b)
            {
                return a.data - b.data;
            }
         });

        // Push the head nodes of all
        // the k lists in 'pq'
        for (int i = 0; i < k; i++)
            if (arr[i] != null)
                pq.add(arr[i]);

        // Loop till 'pq' is not empty
        while (!pq.isEmpty()) 
        {
            // Get the top element of 'pq'
            Node top = pq.peek();
            pq.remove();

            // Check if there is a node
            // next to the 'top' node
            // in the list of which 'top'
            // node is a member
            if (top.next != null)
                // push the next node in 'pq'
                pq.add(top.next);

            // if final merged list is empty
            if (head == null) 
            {
                head = top;
  
                // Points to the last node so far 
                // of the final merged list
                last = top;
            }
            else 
            {
                // Insert 'top' at the end of the 
                // merged list so far
                last.next = top;

                // Update the 'last' pointer
                last = top;
            }
        }

        // Head node of the required merged list
        return head;
    }

    // Function to print the singly linked list
    public static void printList(Node head)
    {
        while (head != null) 
        {
            System.out.print(head.data + " ");
            head = head.next;
        }
    }

    // Utility function to create 
    // a new node
    public Node push(int data)
    {
        Node newNode = new Node(data);
        newNode.next = null;
        return newNode;
    }

    public static void main(String args[])
    {
        // Number of linked lists
        int k = 3; 
    
        // Number of elements in each list
        int n = 4; 

        // An array of pointers storing the 
        // head nodes of the linked lists
        Node arr[] = new Node[k];

        arr[0] = new Node(1);
        arr[0].next = new Node(3);
        arr[0].next.next = new Node(5);
        arr[0].next.next.next = new Node(7);

        arr[1] = new Node(2);
        arr[1].next = new Node(4);
        arr[1].next.next = new Node(6);
        arr[1].next.next.next = new Node(8);

        arr[2] = new Node(0);
        arr[2].next = new Node(9);
        arr[2].next.next = new Node(10);
        arr[2].next.next.next = new Node(11);

        // Merge all lists
        Node head = mergeKSortedLists(arr, k);
        printList(head);
    }
}

class Node 
{
    int data;
    Node next;
    Node(int data)
    {
        this.data = data;
        next = null;
    }
}
// This code is contributed by Gaurav Tiwari