input: Write a function to convert HTML to plain text without using any libraries. The function should remove all HTML tags and return the plain text content.
  
  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>Hello, <b>world</b>!</p>") == "Hello, world!"
  assert html_to_text("<div><h1>Title</h1><p>This is a paragraph.</p></div>") == "TitleThis is a paragraph."
  assert html_to_text("<a href='example.com'>Link</a>") == "Link"
  assert html_to_text("<br>") == ""
  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