leetcode[23]Merge k Sorted Lists

/**
* 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;
}
};