Merge k Sorted Lists

给k个sorted list, 然后merge. 用一个heap存每个list的当前元素. 然后组一个list.

/**
 * 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.length == 0 || lists == null)
            return null;
        PriorityQueue<ListNode> q = new PriorityQueue<ListNode>(lists.length, new Comparator<ListNode>(){
             public int compare(ListNode a, ListNode b) {
                if(a.val > b.val)
                    return 1;
                else if(a.val < b.val)
                    return -1;
                else
                    return 0;
            }
        });
        for(int i = 0 ; i < lists.length; i++) {
            if(lists[i] != null)
                q.add(lists[i]);
        }
        ListNode head = new ListNode(0);
        ListNode p = head;
        while(!q.isEmpty()){
            ListNode node = q.poll();
            p.next = node;
            if(node.next != null)
                q.add(node.next);
            p = p.next;
        }
        return head.next;
    }
}