LeetCoder - 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
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode sortedArrayToBST(int[] num) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(num.length==0) {
            return null;
        }
        return buildTree(num, 0, num.length-1);
    }
   
    private TreeNode buildTree(int[] num, int start, int end) {
        int mid = (start+end)/2;
        TreeNode root = new TreeNode(num[mid]);
        if(mid!=start) {
            root.left = buildTree(num, start, mid-1);
        }
        if(mid!=end) {
            root.right = buildTree(num, mid+1, end);
        }
        return root;
    }
}