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);