input: Write a python function to find the height of a binary tree.
  assert find_height(TreeNode(1)) == 1
  assert find_height(TreeNode(1, TreeNode(2), TreeNode(3))) == 2
  assert find_height(TreeNode(1, TreeNode(2, TreeNode(4)), TreeNode(3))) == 3
output: def find_height(root):
      if not root:
          return 0
      left_height = find_height(root.left)
      right_height = find_height(root.right)
      return max(left_height, right_height) + 1