python3 二叉树的存储和三种遍历

#coding:utf-8
class node():
    def __init__(self,k=None,l=None,r=None):
        self.key=k;
        self.left=l;
        self.right=r;  

def create(root):
    a=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,end='');
        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,end='');
        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,end='');  

root=None;     # 测试代码
root=create(root);
preorder(root);
print('\n')
inorder(root);
print('\n')
postorder(root);