input: Write a function to convert an HTML string to plain text without using any libraries. If the input HTML string is empty, return an empty string.
  
  def html_to_text(html_string):
      output = ''
      in_tag = False
      for char in html_string:
          if char == '<':
              in_tag = True
          elif char == '>':
              in_tag = False
          elif in_tag == False:
              output += char
      return output
  
  assert html_to_text("<p>This is a paragraph.</p>")=="This is a paragraph."
  assert html_to_text("<h1>Heading</h1>")=="Heading"
  assert html_to_text("<div><p>This is in a div.</p></div>")=="This is in a div."
  assert html_to_text("<ul><li>Item 1</li><li>Item 2</li></ul>")=="Item 1Item 2"
  assert html_to_text("")==""
output: def html_to_text(html_string):
      output = ''
      in_tag = False
      for char in html_string:
          if char == '<':
              in_tag = True
          elif char == '>':
              in_tag = False
          elif in_tag == False:
              output += char
      return output