23. Merge k Sorted Lists

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if (lists==null||lists.length==0) return null;
        
        PriorityQueue<ListNode> queue= new PriorityQueue<ListNode>(lists.length,new Comparator<ListNode>(){
            @Override
            public int compare(ListNode o1,ListNode o2){
                
                    return o1.val - o2.val;
            }
        });
        
        ListNode dummy = new ListNode(0);
        ListNode tail=dummy;
        
        for (ListNode node:lists)
            if (node!=null)
                queue.add(node);
            
        while (!queue.isEmpty()){
            tail.next=queue.poll();
            tail=tail.next;
            
            if (tail.next!=null)
                queue.add(tail.next);
        }
        return dummy.next;
    }
}