Function postorder

ParseTree.py:51–55

(tree)

# 递归实现树的后序遍历
def postorder(tree):
    if tree != None:
        postorder(tree.getLeftChild())
        postorder(tree.getRightChild())
        print(tree.getRootVal())

# 利用后序遍历实现两个叶结点的运算
def postordereval(tree):