class Node:
    def __init__(self, value, left=None, right=None):
        self.value = value
        self.left = left
        self.right = right


def postorder(node, result=None):
    if result is None:
        result = []
    if node is None:
        return result
    postorder(node.left, result)
    postorder(node.right, result)
    result.append(node.value)
    return result


# Tree: root 5; left child 3, right child 8.
# 3's children are 2 and 4; 8's children are 7 and 9.
root = Node(5,
            Node(3, Node(2), Node(4)),
            Node(8, Node(7), Node(9)))

print(postorder(root))
# [2, 4, 3, 7, 9, 8, 5]