text: class Solution(object):
      def XXX(self, n):
          """
          :type n: int
          :rtype: int
          """
          if n <= 3 :return n
          test = [0, 1, 2, 3]
          for i in range(n - 3):test.append(test[-1]+test[-2])
          return test[-1]
  
  