leetcode 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) {
        if (num==null || num.length==0) {
            return null;
        }
      int middle = num.length/2;  
      int[] firstarray= new int[middle];
      int[] secondarray = new int[num.length-1-middle];
      for (int i = 0; i < middle; i++) {
        firstarray[i]=num[i];
    }
      for (int i = 0; i <secondarray.length; i++) {
        secondarray[i]=num[middle+1+i];
    }
      TreeNode rootNode=new TreeNode(num[middle]);
      rootNode.left=sortedArrayToBST(firstarray);
      rootNode.right=sortedArrayToBST(secondarray);
      return rootNode;
    }
}