89. Gray Code

class Solution(object):
    def grayCode(self, n):
        """
        :type n: int
        :rtype: List[int]
        """
        if n==0: return [0]
        last_level=self.grayCode(n-1)
        return last_level+map(lambda x:x+(1<<(n-1)),last_level[::-1])