89. Gray Code

https://leetcode.com/problems/gray-code/#/description

The gray code is a binary numeral system where two successive values differ in only one bit.

Given a non-negative integer n representing the total number of bits in the code, print the sequence of gray code. A gray code sequence must begin with 0.

For example, given n = 2, return [0,1,3,2]. Its gray code sequence is:

00 - 0
01 - 1
11 - 3
10 - 2

Note:
For a given n, a gray code sequence is not uniquely defined.

For example, [0,2,3,1] is also a valid gray code sequence according to the above definition.

For now, the judge is able to judge based on one instance of gray code sequence. Sorry about that.

Sol:

class Solution(object):
    def grayCode(self, n):
        """
        :type n: int
        :rtype: List[int]
        """
        # recursive 
        
        # base case
        if n < 1:
            return [0]
        if n == 1:
            return [0, 1]
        
        # recursive case
        res = self.grayCode(n-1)
        
        digit = pow(2, n-1)
        for i in range(digit-1, -1, -1):
            res.append(res[i] + digit)
        
        return res