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.

Solution: Recursion. Pre-order. O(n)

/**
 * 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 *sortedListToBST(ListNode *head) {
        return sortedListToBSTRe(head, getLength(head));
    }
    
    TreeNode* sortedListToBSTRe(ListNode* &head, int length)
    {
        if(length == 0) return NULL;
        int mid = length / 2;
        TreeNode* left = sortedListToBSTRe(head, mid);
        TreeNode* root = new TreeNode(head->val);
        TreeNode* right = sortedListToBSTRe(head->next, length - mid - 1);
        root->left = left;
        root->right = right;
        head = head->next;
        return root;
    }
    
    int getLength(ListNode* head) 
    {
        int length = 0;
        while(head) {
            length++;
            head = head->next;
        }
        return length;
    }
};