* Definition for singly-linked list.
 * public class ListNode {
 * int val;
 * ListNode next;
 * ListNode(int x) { val = x; }
 * }
 */
class  {
    public ListNode mergeKLists(ListNode[] lists) {
        PriorityQueue<ListNode> q = new PriorityQueue<>(new Comparator<ListNode>() {
            @Override
            public int compare(ListNode a, ListNode b) {
                return a.val - b.val;
            }
        });
        for (int i = 0; i < lists.length; i++) {
            if (lists[i] != null) {
                q.offer(lists[i]);
            }
        }
        ListNode res = null;
        ListNode tail = null;
        while (!q.isEmpty()) {
            ListNode tmp = q.poll();
            if (res == null) {
                res = tmp;
                tail = tmp;
            } else {
                tail.next = tmp;
                tail = tmp;
            }
            if (tmp.next != null) {
                q.offer(tmp.next);
            }
        }
        return res;
    }
}