23.合并k个升序链表

和链表一样，就多一个堆而已

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
 struct cmp
{
	bool operator()(ListNode* a, ListNode* b)
	{
		return a->val>b->val;
	}
};

ListNode* mergeKLists(vector<ListNode*>& lists) {
	priority_queue<ListNode*, vector<ListNode*>, cmp>a;
	if(lists.empty())
		return nullptr;
	for(auto listVec : lists)
        if(nullptr!=listVec)
		a.push(listVec);
	ListNode head = ListNode(-1);
	auto node = &head;
	while(!a.empty())
	{
		node->next = a.top();
		if(a.top()->next != nullptr)
			a.push(a.top()->next);
		a.pop();
		node = node->next;
	}
	return head.next;
}

};