class Node:
#构造函数，内置了data参数
def __init__(self, data=""):
self.data = data
#左右孩子都为None
self.lc = None
self.rc = None
#前序遍历
def pre_order(a):
if(a):
print(a.data, end = " ")
if(a.lc): pre_order(a.lc)
if(a.rc): pre_order(a.rc)
#分别创建对应的节点
a = Node("a")
b = Node("b")
c = Node("c")
d = Node("d")
e = Node("e")
f = Node("f")
g = Node("g")
#指定各自的左右孩子
a.lc = b
a.rc = c
b.lc = d
b.rc = e
c.lc = f
c.rc = g
pre_order(a)
