LeetCode 23. Merge k Sorted Lists

https://leetcode.com/problems/merge-k-sorted-lists/description/
Use max heap, the time complexity is O( log(k) * N )

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        if (lists.empty())
            return nullptr;
        auto comparor = [](ListNode* lhs, ListNode* rhs) {
            return lhs->val > rhs->val;
        };
        priority_queue<ListNode*, vector<ListNode*>, decltype(comparor)> pri_queue(comparor);
        // perpare queue with K items
        for(auto& list : lists)
        {
            if (list != nullptr)
                pri_queue.push(list);
        }
        ListNode* head = new ListNode(-1);
        ListNode* prev = head;
        ListNode* current = nullptr;
        while (!pri_queue.empty())
        {
            current = pri_queue.top();
            prev->next = current;
            ListNode* next = current->next;
            pri_queue.pop();
            if (next != nullptr)
                pri_queue.push(next);
            prev = current;
        }
        return head->next;
    }
};