input: Write a function to convert an HTML string to plain text without using any libraries. If the given HTML string is empty then return an empty string.
  assert html_to_text("<p>Hello <b>World</b>!</p>")=="Hello World!"
  assert html_to_text("<h1>Title</h1>")=="Title"
  assert html_to_text("<a href='#'>Link</a>")=="Link"
  assert html_to_text("Hello")=="Hello"
  assert html_to_text("")==""
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
              i += 1
          else:
              plain_text += html[i]
              i += 1
      return plain_text