The k-th Lexicographical String of All Happy Strings of Length n — Python

# Time:  O(n)
# Space: O(1)

class Solution(object):
    def getHappyString(self, n, k):
        """
        :type n: int
        :type k: int
        :rtype: str
        """
        base = 2**(n-1)
        if k > 3*base:
            return ""
        result = [chr(ord('a')+(k-1)//base)]
        while base > 1:
            k -= (k-1)//base*base
            base //= 2
            result.append(('a' if result[-1] != 'a' else 'b') if (k-1)//base == 0 else
                          ('c' if result[-1] != 'c' else 'b'))
        return "".join(result)