input: Write a function to clear all nodes of a binary tree using recursion.
  assert clearBinaryTree(root1) == None
  assert clearBinaryTree(root2) == None
  assert clearBinaryTree(root3) == None
output: def clearBinaryTree(node):
      if node:
          clearBinaryTree(node.left)
          clearBinaryTree(node.right)
          node.left = None
          node.right = None