leetcode[23]Merge k Sorted Lists

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

/**

 * Definition for singly-linked list.

 * struct ListNode {

 *     int val;

 *     ListNode *next;

 *     ListNode(int x) : val(x), next(NULL) {}

 * };

 */

bool comp(ListNode *a, ListNode *b)

{

    return a->val>b->val;

}

class Solution {  

public: 

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

{

    int n=lists.size();

    if(lists.empty())return NULL;

    vector<ListNode *> v;

    v.reserve(n);

    for (int i=0;i<n;i++)

    {

       if(lists[i]!=NULL)

       v.push_back(lists[i]); 

    }

    if(v.size()==0)return NULL;

    make_heap(v.begin(),v.end(),comp);

    pop_heap(v.begin(),v.end(),comp);

    ListNode *small=v.back();

    v.pop_back();

    ListNode *head=new ListNode(-1);

    ListNode *pL=head;

     pL->next=small;

    pL=pL->next;

    while (!v.empty())

    {

        if (small->next!=NULL)

        {

            v.push_back(small->next);

            push_heap(v.begin(),v.end(),comp);

        }

        pop_heap(v.begin(),v.end(),comp);

        small=v.back();

        v.pop_back();

        pL->next=small;

        pL=pL->next;

    }

    return head->next;

}  

};