2309. Greatest English Letter in Upper and Lower Case

class Solution:
    def greatestLetter(self, s: str) -> str:
        mask1 = mask2 = 0
        for c in s:
            if c.islower():
                mask1 |= 1 << (ord(c) - ord("a"))
            else:
                mask2 |= 1 << (ord(c) - ord("A"))
        mask = mask1 & mask2
        return chr(mask.bit_length() - 1 + ord("A")) if mask else ""