Leetcode - Tree - 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 binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode *sortedArrayToBST(vector<int> &num) {
        if(num.empty())
            return NULL;
        int mid=(num.size()-1)/2;
        TreeNode *root=new TreeNode(num[mid]);
        vector<int> leftArray(num.begin(),num.begin()+mid);
        vector<int> rightArray(num.begin()+mid+1,num.end());
        if(mid>0)
            root->left=sortedArrayToBST(leftArray);
        root->right=sortedArrayToBST(rightArray);
        return root;
    }
};