Convert Sorted List to Binary Search Tree

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
private:
vector<ListNode*> ptrs;
void buildTree(int start,int end,TreeNode* &parent)
{
if(start>=end)
{
return;
}
int mid=start+(end-start)/2;
TreeNode* child=new TreeNode(ptrs[mid]->val);
parent=child;
buildTree(start,mid,child->left);
buildTree(mid+1,end,child->right);
}
public:
TreeNode *sortedListToBST(ListNode *head) {
while(head){
ptrs.push_back(head);
head=head->next;
}
if(ptrs.size()==0)
{
return NULL;
}
int mid=ptrs.size()/2;
TreeNode* root=new TreeNode(ptrs[mid]->val);
buildTree(0,mid,root->left);
buildTree(mid+1,ptrs.size(),root->right);
return root;
}
};