合并k个升序链表

/**
 * 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) {}
 * };
 */
struct heap{
    int val,id;
    heap(){}
    heap(int _val,int _id):val(_val),id(_id){}
    friend bool operator <(const heap b,const heap a){
        return a.val<b.val;
    } 
};
class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        const int N=lists.size();
        if(!N) return nullptr;
        ListNode* nxtNode[N];
        priority_queue<heap> h;
        for(int i=0;i<N;++i){
            nxtNode[i]=lists[i];
            if(nxtNode[i]==nullptr) continue;
            h.push(heap(nxtNode[i]->val,i));
            nxtNode[i]=nxtNode[i]->next;
        }
        heap top;
        ListNode *head=nullptr,*lstNode=nullptr;
        while(!h.empty()){
            top=h.top();h.pop();
            ListNode *now=new ListNode(top.val,nullptr);
            if(lstNode==nullptr) head=now;
            else lstNode->next=now;
            lstNode=now;
            if(nxtNode[top.id]!=nullptr){
                h.push(heap(nxtNode[top.id]->val,top.id));
                nxtNode[top.id]=nxtNode[top.id]->next;
            }
        }
        return head;
    }
};