Convert Sorted Array to Binary Search 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 *convertBinaryTree(vector<int> num, int low, int upper){
       TreeNode *node;
       int mid = (low+upper)/2;
       node = new TreeNode(num[mid]);
       if(low>upper) return NULL;
       if(low==upper) return node;
       else{
         node->left = convertBinaryTree(num, low, mid-1);
         node->right = convertBinaryTree(num, mid+1, upper);
       }
       return node;
    }
    TreeNode *sortedArrayToBST(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int size = num.size();
        TreeNode *node;
        if(size==0) return NULL;
        int low = 0;
        int upper = size-1;
        return convertBinaryTree(num, low, upper);  
    }
};