python递归实现二叉树的三种遍历

#difination of a binary tree
class Node:
    def __init__(self,value=None,left=None,right=None):
        self.value=value
        self.left=left
        self.right=right

def preTraverse(root):
    # 前序遍历
    if root==None:
        return
    print(root.value)
    preTraverse(root.left)
    preTraverse(root.right)

def midTraverse(root):
    #中序遍历
    if root==None:
        return
    midTraverse(root.left)
    print(root.value)
    midTraverse(root.right)

def afterTraverse(root):
    #后序遍历
    if root==None:
        return
    afterTraverse(root.left)
    afterTraverse(root.right)
    print(root.value)

if __name__=='__main__':
    root=Node('D',Node('B',Node('A'),Node('C')),Node('E',right=Node('G',Node('F'))))
    print('前序遍历：')
    preTraverse(root)
    print('\n')
    print('中序遍历：')
    midTraverse(root)
    print('\n')
    print('后序遍历：')
    afterTraverse(root)
    print('\n')