input: Write a function to find the depth of a binary tree.
  assert findDepth(Node(1, Node(2), Node(3, Node(4), Node(5)))) == 3
  assert findDepth(Node(1, Node(2, Node(4)), Node(3))) == 3
  assert findDepth(Node(1)) == 1
output: class Node:
      def __init__(self, value, left=None, right=None):
          self.value = value
          self.left = left
          self.right = right
  
  def findDepth(root):
      if root is None:
          return 0
      else:
          left_depth = findDepth(root.left)
          right_depth = findDepth(root.right)
          return max(left_depth, right_depth) + 1