Merge k Sorted Lists

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

public class Solution {
    public ListNode mergeKLists(ArrayList<ListNode> lists) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(lists.size()==0) return null;
        PriorityQueue<ListNode> q = new PriorityQueue<ListNode>(lists.size(),new Comparator<ListNode>(){
            public int compare(ListNode a, ListNode b){
                return a.val>b.val?1:(a.val==b.val?0:-1);
                }
            });
        
        for(ListNode list:lists){
            if(list!=null) q.add(list);
        }
                
        ListNode head = new ListNode(0), prev = head;
        while(q.size()!=0){
            ListNode temp = q.poll();
            prev.next = temp;
            if(temp.next!=null) q.add(temp.next);
            prev = prev.next;
        }
        return head.next;
    }
}

 第二遍：

public class Solution {
    public ListNode mergeKLists(ArrayList<ListNode> lists) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(lists.size()==0) return null;
        ListNode header = new ListNode(-1), cur = header;
        while(true){
            int pos = -1;
            for(int i = 0; i < lists.size(); i ++){
                if(lists.get(i) != null && (pos == -1 || lists.get(i).val < lists.get(pos).val))
                    pos = i;
            }
            if(pos != -1){
                cur.next = lists.get(pos);
                cur = cur.next;
                lists.set(pos,lists.get(pos).next);
            }else{
                break;
            }
        }
        return header.next;
    }
}