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('abcaaa')=='1a1b1c3a'
  assert compress('zzzzzz')=='6z'
  assert compress('')==''
  assert compress('abc')=='1a1b1c'
output: def compress(s):
      if not s:
          return ''
      result = []
      count = 1
      for i in range(1, len(s)):
          if s[i] == s[i-1]:
              count += 1
          else:
              result.append(f"{count}{s[i-1]}")
              count = 1
      result.append(f"{count}{s[-1]}")
      return ''.join(result)