python中创建和遍历二叉树

#!/usr/bin/python 
class node():
	def __init__(self,k=None,l=None,r=None):
		self.key=k;
		self.left=l;
		self.right=r;

def create(root):
	a=raw_input('enter a key:');
	if a is '#':
		root=None;
	else:
		root=node(k=a);
		root.left=create(root.left);
		root.right=create(root.right);
	return root;

def preorder(root):      #前序遍历
	if root is None:
		return ;
	else :
		print root.key;
		preorder(root.left);
		preorder(root.right);

def inorder(root):     #中序遍历
	if root is None:
		return ;
	else:
		inorder(root.left);
		print root.key;
		inorder(root.right);

def postorder(root):   # 后序遍历
	if root is None:
		return ;
	else :
		postorder(root.left);
		postorder(root.right);
		print root.key;
		
root=None;     # 测试代码
root=create(root);
preorder(root);
inorder(root);
postorder(root);