Convert Sorted List to Binary Search Tree

class Solution {
public:
    TreeNode *sortedListToBST(ListNode *head) {
        ListNode *p;
        int count=0;
        p=head;
        while(p!=NULL)
        {
            count++;
            p=p->next;
        }
        return sorta(head,0,count-1);
    }
    TreeNode *sorta(ListNode *&head,int start,int end)
    {
        if(start>end)
            return NULL;
        int mid=(start+end)/2;
        TreeNode *left=sorta(head,start,mid-1);
        TreeNode *root=new TreeNode(head->val);
        head=head->next;
        TreeNode *right=sorta(head,mid+1,end);
        root->left=left;
        root->right=right;
        return root;
    }
};