[leetcode]23. Merge k Sorted Lists

链接：https://leetcode.com/problems/merge-k-sorted-lists/description/

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

Example:

Input:
[
  1->4->5,
  1->3->4,
  2->6
]
Output: 1->1->2->3->4->4->5->6

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    struct cmp{
        bool operator()(const ListNode* a,const ListNode* b)
        {
            return a->val>b->val;
        }
    };
    
    
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        priority_queue<ListNode*,vector<ListNode*>,cmp> pq;
        for(auto l: lists)
        {
            if(l)
            {
                pq.push(l);
            }
        }
        
        if(pq.empty())
        {
            return nullptr;
        }
        
        ListNode* ans=pq.top();
        pq.pop();
        ListNode* tail=ans;
        if(tail->next)
        {
            pq.push(tail->next);
        }
        
        while(!pq.empty())
        {
            tail->next=pq.top();
            tail=tail->next;
            pq.pop();
            if(tail->next)
            {
                pq.push(tail->next);
            }
        }
        return ans;
    }
};