Calculate Size of a Binary Tree

class Stack(object):
    def __init__(self):
        self.items = []

    def __len__(self):
        return self.size()

    def size(self):
        return len(self.items)

    def push(self, item):
        self.items.append(item)

    def pop(self):  
        if not self.is_empty():
            return self.items.pop()

    def peek(self):
        if not self.is_empty():
            return self.items[-1]

    def is_empty(self):
        return len(self.items) == 0


class Node(object):

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

class BinaryTree(object):

    def __init__(self, root):

        self.root = Node(root)

    def size(self):

        if self.root is None:

            return 0

        stack = Stack()

        stack.push(self.root)

        size = 1

        while stack:

            node = stack.pop()

            if node.left:

                size += 1

                stack.push(node.left)

            if node.right:

                size += 1

                stack.push(node.right)

        return size


    def size_recursive(self, node):
        if node is None:
            return 0
        return 1 + self.size_recursive(node.left) + self.size_recursive(node.right)



tree = BinaryTree(1)
tree.root.left = Node(2)
tree.root.right = Node(3)
tree.root.left.left = Node(4)
tree.root.left.right = Node(5)



print(tree.size_recursive(tree.root))


print(tree.size())