Merge K sorted linked lists

ListNode.java

package com.interviewbytes.linkedlists;


public class ListNode {
    int val;
    ListNode next;

    ListNode(int x) {
        val = x;
    }
}

MergeKLists.java

package com.interviewbytes.linkedlists;


import java.util.Comparator;
import java.util.PriorityQueue;

public class MergeKLists {

    private static final Comparator<ListNode> COMPARATOR = new Comparator<ListNode>() {
        @Override
        public int compare(ListNode o1, ListNode o2) {
            return o1.val - o2.val;
        }
    };

    public ListNode mergeKLists(ListNode[] lists) {

        if (lists == null || lists.length == 0) return null;

        PriorityQueue<ListNode> priorityQueue = new PriorityQueue<>(COMPARATOR);

        for (ListNode listNode : lists) {
            if (listNode != null) priorityQueue.offer(listNode);
        }

        ListNode sentinel = new ListNode(0);
        ListNode current = sentinel;

        while (!priorityQueue.isEmpty()) {
            ListNode node = priorityQueue.poll();
            current.next = node;
            current = current.next;

            if (node.next != null) priorityQueue.offer(node.next);
        }

        return sentinel.next;
    }
}