Convert Sorted List to Binary Search Tree

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

/**
* 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* createNode(ListNode *head, int len)
{
if (head == NULL || len < 1)
{
return NULL;
}

if (len == 1)
{
return new TreeNode(head->val);
}

ListNode *p = head;

int steps = len/2 - 1;
int leftLen = steps + 1;
int rightLen = len - leftLen - 1;
while (steps)
{
steps--;
p = p->next;
}

TreeNode *result = new TreeNode(p->next->val);
ListNode *temp = p->next->next;
p->next = NULL;
result->left = createNode(head, leftLen);
result->right = createNode(temp, rightLen);

return result;
}

TreeNode *sortedListToBST(ListNode *head) {
if (head == NULL)
{
return NULL;
}

int len = 0;
ListNode *p = head;
while (p)
{
len++;
p = p->next;
}

return createNode(head, len);
}
};