二叉树中序遍历

public List inorderTraversal(TreeNode root) {
        List list = new ArrayList();
        inTraversal(root, list);
        return list;
    }
    
    void inTraversal (TreeNode node,List list) {
        if (node == null) {
            return;
        }
        if (node.left != null) {
            inTraversal(node.left, list);
        }
        list.add(node.val);
        if (node.right != null) {
            inTraversal(node.right, list);
        }
    }