input: Write a python function to find the maximum value in a binary tree.
  assert max_in_tree(Node(1, Node(2), Node(3))) == 3
  assert max_in_tree(Node(10, Node(5, Node(3), Node(7)), Node(20, None, Node(30)))) == 30
  assert max_in_tree(Node(2, Node(1), Node(4, Node(3), Node(5)))) == 5
output: class Node:
      def __init__(self, value=0, left=None, right=None):
          self.value = value
          self.left = left
          self.right = right
  
  def max_in_tree(root):
      if root is None:
          return float('-inf')
      left_max = max_in_tree(root.left)
      right_max = max_in_tree(root.right)
      return max(root.value, left_max, right_max)