leetcode专题训练 89. Gray Code

很简单的一道题，找规律就好。

class Solution:
    def grayCode(self, n: int) -> List[int]:
        if n == 0:
            return [0]
        if n == 1:
            return [0, 1]
        code = [0, 1]
        addnum = 2
        for i in range(n-1):
            tmpcode = [code[j]+addnum for j in range(len(code))]
            code += tmpcode[::-1]
            addnum <<= 1
        return code