Convert Sorted Array to Binary Search Tree — C++

// Time:  O(n)
// Space: O(logn)

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* sortedArrayToBST(vector<int>& nums) {
        return sortedArrayToBSTHelper(nums, 0, nums.size() - 1);
    }

private:
    TreeNode *sortedArrayToBSTHelper(vector<int> &nums, int start, int end) {
        if (start <= end) {
            TreeNode *node = new TreeNode(nums[start + (end - start) / 2]);
            node->left = sortedArrayToBSTHelper(nums, start, start + (end - start) / 2 - 1);
            node->right = sortedArrayToBSTHelper(nums, start + (end - start) / 2 + 1, end);
            return node;
        }
        return nullptr;
    }
};