input: 
  def compress(s: str) -> str:
      """ Compress a given string by replacing all repeating characters with a single 
      instance and its count.
      >>> compress("aabbbbbbcccc")
      '2a6b4c'
      >>> compress("aabbcc")
      '2a2b2c'
      >>> compress("abcd")
      '1a1b1c1d'
      """
  
output:     compressed = ""
      count = 1
      for i in range(1, len(s)):
          if s[i] == s[i - 1]:
              count += 1
          else:
              compressed += str(count) + s[i - 1]
              count = 1
      compressed += str(count) + s[-1]
      return compressed