struct Node

{

	ListNode* l;

	bool operator < (const Node& n) const {

		return l->val > n.l->val;

	}

    // 这里有两种写法

	// friend bool operator < ( Node a,  Node b) {

	// 	return a.l->val > b.l->val;

	// }

};

class Solution {

public:

	ListNode* mergeKLists(vector<ListNode*>& lists) {

		priority_queue<Node> q;

		for (ListNode* p : lists) {

            if(p){

			    q.push({ p });

            }

		}

		ListNode head, * tail = &head;

		while (!q.empty()) {

			auto temp = q.top();

			q.pop();

			if (temp.l->next) {

				q.push({ temp.l->next });

			}

			tail->next = temp.l;

			tail = tail->next;

			tail->next = nullptr;

		}

		return head.next;

	}

};