LeetCode 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 {
public:
   TreeNode *getbTree(ListNode *head,int start,int end)
    {
        if(start<=end)
        {
            int mid = (start+end)>>1;
            ListNode *work = head;
            ListNode *rstart=NULL;
            for(int i=start;i<mid;i++)
            {
                work = work->next;
            }
            rstart = work->next;
            TreeNode *node = new TreeNode(work->val);
            node->left = getbTree(head,start,mid-1);
            node->right = getbTree(rstart,mid+1,end);
            return node;
        }
        else
            return NULL;
    }
    TreeNode *sortedListToBST(ListNode *head) {
        if(head==NULL)return NULL;
        int len = 0;
        ListNode *p = head;
        while(p)
        {
            ++len;
            p=p->next;
        }

        return getbTree(head,0,len-1);

    }
};