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.
  assert html_to_text("<html><body>Hello, <b>world</b>!</body></html>")=="Hello, world!"
  assert html_to_text("<p>This is a <i>test</i> paragraph.</p>")=="This is a test paragraph."
  assert html_to_text("<div><h1>Title</h1><p>Content</p></div>")=="TitleContent"
  assert html_to_text("<a href='https://www.example.com'>Link</a>")=="Link"
  assert html_to_text("")==""
  assert html_to_text("<br>")==""
output: def html_to_text(html):
      if not html:
          return ""
      plain_text = ""
      i = 0
      while i < len(html):
          if html[i] == '<':
              while i < len(html) and html[i] != '>':
                  i += 1
          else:
              plain_text += html[i]
          i += 1
      return plain_text