NA改日再战1763. 最长的美好子字符串

class Solution:
    def longestNiceSubstring(self, s: str) -> str:
        '''
        思路：子串是连续的，通过将字母变为小写or大写，判断字母的左右两边是否一样即可
        '''
        if not s or len(s) == 1:
            return ''
        res = ''
        for i in range(len(s)-1):
            res += s[i]
            if s[i].upper() !=  s[i+1] or s[i].lower() != s[i+1]:
                res = ''
        return res