code: def num_decodings(s):
      
      if not s or s[0] == "0":
          return 0
      wo_last, wo_last_two = 1, 1
      for i in range(1, len(s)):
          x = wo_last if s[i] != "0" else 0
          y = wo_last_two if int(s[i-1:i+1]) < 27 and s[i-1] != "0" else 0
          wo_last_two = wo_last
          wo_last = x+y
      return wo_last
docs: :type s: str
      :rtype: int