input: Write a function to compress a given string by replacing all repeating characters with a single instance and its count. If the given string is empty, return an empty string.
  assert compress("aabbbbbbcccc")=="2a6b4c"
  assert compress("aaabbbccc")=="3a3b3c"
  assert compress("abcdef")=="1a1b1c1d1e1f"
  assert compress("")==""
  assert compress("a")=="1a"
output: def compress(s):
      if not s:
          return ""
      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)