#include <string>
#include <cctype>
#include <cstdio>

class Solution {
private:
	static bool isNice(const std::string& str, int L, int R) {
		bool bucket[2][26]{};
		for (int i = L; i < R; ++i)
			*(std::isupper(str[i]) ? &bucket[0][str[i] - 'A'] : &bucket[1][str[i] - 'a']) = true;
		for (int i = 0; i < 26; ++i)
			if ((bucket[0][i] || bucket[1][i]) && !(bucket[0][i] && bucket[1][i]))
				return false;
		return true;
	}
public:
    static std::string longestNiceSubstring(const std::string& s) {
		const int n = s.length();
		std::string ret;
		int retMax = 0;
		for (int i = 0; i < n; ++i) {
			for (int j = i + 1; j <= n; ++j) {
				if (j - i > retMax && isNice(s, i, j)) {
					retMax = j - i;
					ret = s.substr(i, j - i);
				}
			}
		}
		return ret;
    }
};

int main() {
	std::printf("%s\n", Solution::longestNiceSubstring("YazaAay").c_str());
	std::printf("%s\n", Solution::longestNiceSubstring("Bb").c_str());
	std::printf("%s\n", Solution::longestNiceSubstring("c").c_str());
	std::printf("%s\n", Solution::longestNiceSubstring("dDzeE").c_str());
	return 0;
}