input: def compress(s):
      """ Compress the given string by replacing all repeating characters with a single
      instance and its count.
      >>> compress("aabbbbbbcccc")
      '2a6b4c'
      """
      output = ""
      count = 1
      for i in range(len(s)):
          if (i+1 < len(s) and s[i] == s[i+1]):
              count += 1
          else:
              output += str(count) + s[i]
              count = 1
      return output
  
  s = "aabbbbbbcccc"
  print(compress(s))
  # Output: 2a6b4c
output:     output = ""
      count = 1
      for i in range(len(s)):
          if (i+1 < len(s) and s[i] == s[i+1]):
              count += 1
          else:
              output += str(count) + s[i]
              count = 1
      return output