input: Write a python function to compute the height of a binary tree.
  assert getHeight(None) == 0
  assert getHeight(TreeNode(1)) == 1
  assert getHeight(TreeNode(1, TreeNode(2), TreeNode(3))) == 2
  assert getHeight(TreeNode(1, TreeNode(2, TreeNode(4)), TreeNode(3))) == 3
output: def getHeight(root):
      if root is None:
          return 0
      left_height = getHeight(root.left)
      right_height = getHeight(root.right)
      return max(left_height, right_height) + 1