String Without AAA or BBB — Python

# Time:  O(a + b)
# Space: O(1)

class Solution(object):
    def strWithout3a3b(self, A, B):
        """
        :type A: int
        :type B: int
        :rtype: str
        """
        result = []
        put_A = None
        while A or B:
            if len(result) >= 2 and result[-1] == result[-2]:
                put_A = result[-1] == 'b'
            else:
                put_A = A >= B

            if put_A:
                A -= 1
                result.append('a')
            else:
                B -= 1
                result.append('b')
        return "".join(result)