input: Write a python function to count the number of nodes in a binary tree.
  assert countNodes(None) == 0
  assert countNodes(TreeNode(1, TreeNode(2), TreeNode(3))) == 3
  assert countNodes(TreeNode(1, TreeNode(2, TreeNode(4), TreeNode(5)), TreeNode(3))) == 5
output: class TreeNode:
      def __init__(self, val=0, left=None, right=None):
          self.val = val
          self.left = left
          self.right = right
  
  def countNodes(root):
      if root is None:
          return 0
      return 1 + countNodes(root.left) + countNodes(root.right)