input: Write a function to convert an HTML string to plain text without using any external libraries. If the input 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>Hello, <b>world</b>!</p>")=='Hello, world!'
  assert html_to_text("<a href='example.com'>Link</a>")=='Link'
  assert html_to_text("<div><p>Paragraph 1</p><p>Paragraph 2</div>")== 'Paragraph 1Paragraph 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