Convert Sorted Array to Binary Search Tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

/**
 * 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) {
        if (nums.empty())
            return nullptr;
        TreeNode *root = helper(nums, 0, nums.size()-1);
        return root;
    }
    TreeNode *helper(vector<int>& nums, int start, int end){
        if (start > end)
            return nullptr;
        int mid = start + (end - start)/2;
        TreeNode *root = new TreeNode(nums[mid]); #指针new
        root->left = helper(nums, start, mid-1);
        root->right = helper(nums, mid+1, end);
        return root;
    }
};