input: Write a function to convert an HTML string to plain text without using any libraries. The function should strip all HTML tags and return the plain text content. If the input HTML string is empty, the function should return an empty string.
  assert html_to_text("<html><body><h1>Hello</h1><p>World</p></body></html>")=="HelloWorld"
  assert html_to_text("<div><span>This is a span</span></div>")=="This is a span"
  assert html_to_text("<a href='example.com'>Link</a>")=="Link"
  assert html_to_text("<p>No tags here</p>")=="No tags here"
  assert html_to_text('')==''
output: def html_to_text(html):
      if not html:
          return ""
      return ''.join(filter(lambda x: x not in '<>', html)).replace('/', '')