input: 
  from typing import List
  
  def compress(s: str) -> str:
      """
      Compress a given string by replacing all repeating characters with a single instance and its count.
      
      Example:
      for compress("aabbbbbbcccc") == '2a6b4c'
      """
  
output:     compressed = []
      count = 1
      for i in range(1, len(s)):
          if s[i] == s[i - 1]:
              count += 1
          else:
              compressed.append(f"{count}{s[i - 1]}")
              count = 1
      compressed.append(f"{count}{s[-1]}")
      return ''.join(compressed)