new_contents: class Solution:
      # @param {int} num a non-negative integer
      # @return {int} one digit
      def addDigits(self, num):
          while num > 9:
              nxt = 0
              
              while num > 0:
                  nxt  += num % 10
                  num //= 10
                  
              num = nxt
                  
          return num
  
subject: Create add_digits2.py