1763. Longest Nice Substring

class Solution:
    def longestNiceSubstring(self, s: str) -> str:
        n = len(s)
        res = ''
        for i in range(n):
            lower = upper = 0
            for j in range(i, n):
                if s[j].islower():
                    lower |= 1 << (ord(s[j]) - ord('a'))
                else:
                    upper |= 1 << (ord(s[j]) - ord('A'))
                if lower == upper and len(res) < j - i + 1:
                    res = s[i : j + 1]
        return res