编一个程序，读入用户输入的一串先序遍历字符串，根据此字符串建立一个二叉树（以指针方式存储）。

 public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        while (scanner.hasNext()){
            String line = scanner.next();
            TreeNode2 root = build(line);
            inOrder(root);
            System.out.println();
        }
    }

    private static int index = 0;

    private static TreeNode2 build(String line) {
        index = 0;
        return creatTreePreOrder(line);
    }

    private static TreeNode2 creatTreePreOrder(String line) {
        char cur = line.charAt(index);
        if (cur == '#'){
            return null;
        }
        TreeNode2 root = new TreeNode2(cur);
        index++;
        root.left = creatTreePreOrder(line);
        index++;
        root.right = creatTreePreOrder(line);
        return root;
    }

    private static void inOrder(TreeNode2 root) {
    }