class Node:             #建立二叉树
    def __init__(self,value=None,left=None,right=None):
        self.value=value
        self.left=left      #左子树
        self.right=right    #右子树

def preTraverse(root):       #前序遍历
    if root==None:
        return
    print(root.value,end="")
    __________
    __________

def midTraverse(root):       #中序遍历
    if root==None:
        return
    __________
    __________
    __________
    
def afterTraverse(root):     #后序遍历
    if root==None:
        return
    __________
    __________
    __________

root=Node('D',Node('B',Node('A'),Node('C')),Node('E',right=Node('G',Node('F'))))
preTraverse(root)
print()
midTraverse(root)
print()
afterTraverse(root)