input: from typing import List
  
  def convert_html_to_text(html_string: str) -> str:
      """ Converts a given HTML string to plain text without using any external libraries.
      It removes all HTML tags and returns only the text content.
      >>> convert_html_to_text('<h1>Hello, World!</h1>')
      'Hello, World!'
      >>> convert_html_to_text('<p>Stay hydrated. <strong>Drink water!</strong></p>')
      'Stay hydrated. Drink water!'
      """
output:     import re
      text = re.sub(r'<[^>]+>', '', html_string)
      return text