input: Write a function to compress a given string by replacing all repeating characters with a single instance and its count. If the string is empty, return an empty string.
  def compress(s):
      if not s:
          return ""
      output = ""
      count = 1
      for i in range(1, len(s)):
          if s[i] == s[i-1]:
              count += 1
          else:
              output += str(count) + s[i-1]
              count = 1
      output += str(count) + s[-1]
      return output
  
  assert compress("aabbbbbbcccc")=="2a6b4c"
  assert compress("a")=="1a"
  assert compress("")==""
  assert compress("aaaaaa")=="6a"
  assert compress("abcccdeee")=="1a1b3c1d3e"
output: def compress(s):
      if not s:
          return ""
      output = ""
      count = 1
      for i in range(1, len(s)):
          if s[i] == s[i-1]:
              count += 1
          else:
              output += str(count) + s[i-1]
              count = 1
      output += str(count) + s[-1]
      return output