Merge k sorted lists

"Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity."

Using a PriorityQueue is a very smart way. The time complexity is n* O(log(lists.size())) where n is the total nodes in all linked lists.

public class ListNode {
     int val;
     ListNode next;
     ListNode(int x) {
         val = x;
         next = null;
     }
 }

public class MergeLists {
    public ListNode mergeKLists(List<ListNode> lists) {
        if (lists == null || lists.size() == 0)
            return null;
        PriorityQueue<ListNode> listNodeQueue = new PriorityQueue<ListNode> (lists.size(), new ListNodeComparator());
        for (ListNode node : lists) {
            if (node != null)
                listNodeQueue.add(node);
        }
        ListNode dummy = new ListNode(0);
        ListNode head = dummy;
        while (!listNodeQueue.isEmpty()) {
            ListNode toAdd = listNodeQueue.poll();
            head.next = toAdd;
            if (toAdd.next != null)
                listNodeQueue.add(toAdd.next);
            head = head.next;
        }
        return dummy.next;
        
    }
    private class ListNodeComparator implements Comparator<ListNode> {
        public int compare (ListNode a, ListNode b) {
            return a.val - b.val;
        }
    }
}