Array to BST

class Solution {
    class Node {
        int data;
        Node left;
        Node right;
        public Node(int data) {
            this.data = data;
            this.left = this.right = null;
        }
    }
    
    public int[] sortedArrayToBST(int[] nums) {
        Node root = sortedArrayToBSTUtil(nums, 0, nums.length - 1);
        int[] index = new int[] {0};
        preOrder(root, nums, index);
        return nums;
    }
    
    private Node sortedArrayToBSTUtil(int[] nums, int start, int end) {
        if(start > end) 
            return null;
        
        int mid = start + (end - start) / 2;
        Node root = new Node(nums[mid]);
        root.left = sortedArrayToBSTUtil(nums, start, mid - 1);
        root.right = sortedArrayToBSTUtil(nums, mid + 1, end);
        
        return root;
    }
    
    private void preOrder(Node root, int[] nums, int[] index) {
        if(root == null)
            return;
        
        nums[index[0]++] = root.data;
        preOrder(root.left, nums, index);
        preOrder(root.right, nums, index);
    }
}