leetcode--Convert Sorted List to Binary Search Tree

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

public class Solution {
    /** Convert the linked list to arrayList first, and then convert the ArrayList to a BST
	 * @param head  --head node of the linked list
	 * @return TreeNode --the root node of a height balanced BST tree
	 * @author Averill Zheng
	 * @version 2014-06-05
	 * @since JDK 1.7 
	 */
	public TreeNode sortedListToBST(ListNode head) {
		TreeNode root = null;
		ArrayList<Integer> number = new ArrayList<Integer>();
		int length = 0;
		while(head != null){
			number.add(head.val);
			head = head.next;
			++length;
		}
		root = convertHelper(number, 0, length);
		return root;
    }
	private TreeNode convertHelper(ArrayList<Integer> number, int i, int j){
		TreeNode root = null;
		if(j > i){
			if(j == i + 1)
				root = new TreeNode(number.get(i));
			else{
				int mid = i + (j - i) / 2;
				root = new TreeNode(number.get(mid));
				root.left = convertHelper(number,i, mid);
				root.right = convertHelper(number, mid + 1, j);
			}
		}
		return root;    
    }
}