Leetcode-Convert Sorted Array to BST

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

Solution:

/**
 * 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;
        
        int end = num.length-1;
        TreeNode root = sortedArrayToBSTRecur(num,0,end);
        return root;
    }
    
    public TreeNode sortedArrayToBSTRecur(int[] num, int head, int end){
        if (head==end){
            TreeNode root = new TreeNode(num[head]);
            return root;
        }
        
        if (head+1==end){
            TreeNode root = new TreeNode(num[end]);
            TreeNode child = new TreeNode(num[head]);
            root.left = child;
            return root;
        }
        
        //Calculate the median index.
        int len = end-head;
        int mid = head + len/2 + len%2;
        TreeNode root = new TreeNode(num[mid]);
        TreeNode leftChild = sortedArrayToBSTRecur(num,head,mid-1);
        TreeNode rightChild = sortedArrayToBSTRecur(num,mid+1,end);
        root.left = leftChild;
        root.right = rightChild;
        return root;
    }
}