Convert Sorted Array to Binary Search Tree

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

Solution:
Basic recursion question. 

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode sortedArrayToBST(int[] num) {
        if (num.length == 0) return null;
        if (num.length == 1) return new TreeNode(num[0]);
        return buildBST(num, 0, num.length-1);
    }
   
    public TreeNode buildBST(int[] num, int first, int last){
        if(first <= last){
            int mid = (first + last)/2;
            TreeNode root = new TreeNode(num[mid]);
            root.left = buildBST(num, first, mid-1);
            root.right = buildBST(num, mid+1, last);
            return root;
        }
        return null;
    }
}