23. Merge k Sorted Lists via Java

class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if(lists==null || lists.length==0){
            return null;
        }
        ListNode res = new ListNode(0);
        ListNode cur = res;
        // lambda instead of anonymous comparator class
        PriorityQueue<ListNode> pq = new PriorityQueue<>((ListNode l1, ListNode l2)->l1.val-l2.val);
        // classic comparator
        // PriorityQueue<ListNode> pq = new PriorityQueue<>(new Comparator<ListNode>(){
        //     @Override 
        //     public int compare(ListNode i, ListNode j) { 
        //         if(i.val==j.val){ 
        //             return 0;
        //         }
        //         return i.val<j.val?-1:1;
        //     }});
        for(int i = 0; i<lists.length; i++){
            if(lists[i]!=null){
                pq.add(lists[i]);
            }
        }
        while(!pq.isEmpty()){
            cur.next = pq.poll();
            if(cur.next.next!=null){
                pq.offer(cur.next.next);
            }
            cur = cur.next;
        }
        cur.next = null;
        return res.next;
    }
}