input: Write a function to compress a given string by replacing all repeating characters with a single instance and its count.
  
  def compress(s): 
      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
  
  assert compress("aabbbbbbcccc") == "2a6b4c"
  assert compress("aabbcc") == "2a2b2c"
  assert compress("abcdef") == "1a1b1c1d1e1f"
  assert compress("a") == "1a"
  assert compress("") == ""
output: def compress(s): 
      if not s:
          return ""
      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