LeetCode Merge k Sorted Lists

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

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode mergeKLists(List<ListNode> lists) {
        if (lists == null || lists.size() == 0) {
            return null;
        }
        Comparator<ListNode> compare=new Comparator<ListNode>() {
            @Override
            public int compare(ListNode o1, ListNode o2) {
                return o1.val - o2.val;
            }
        };

        PriorityQueue<ListNode> queue = new PriorityQueue<ListNode>(lists.size(), compare);
        for (ListNode node : lists) {
            if (node != null) {
                queue.add(node);
            }
        }
        ListNode ret = new ListNode(0);
        ListNode temp = ret;
        while (!queue.isEmpty()) {
            temp.next=queue.poll();
            temp=temp.next;
            if (temp.next != null) {
                queue.add(temp.next);
            }
        }
        return ret.next;
    } 
    
}