Convert Sorted Array to Binary Search Tree

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public TreeNode sortedArrayToBST2(int[] num, int first, int last){        if(last < first)return null;                int middle = (first+last+1)/2;                TreeNode head = new TreeNode(num[middle]);        head.left = sortedArrayToBST2(num, first, middle - 1);        head.right = sortedArrayToBST2(num, middle + 1, last);        return head;            }    public TreeNode sortedArrayToBST(int[] num) {       return sortedArrayToBST2(num, 0 , num.length-1);            }}