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);