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Java-based LeetCode algorithm problem solutions, regularly updated

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package g1701_1800.s1763_longest_nice_substring;

// #Easy #String #Hash_Table #Bit_Manipulation #Sliding_Window
// #2022_04_30_Time_5_ms_(61.88%)_Space_43.7_MB_(27.43%)

import java.util.HashSet;
import java.util.Set;

/**
 * 1763 - Longest Nice Substring\.
 *
 * Easy
 *
 * A string `s` is **nice** if, for every letter of the alphabet that `s` contains, it appears **both** in uppercase and lowercase. For example, `"abABB"` is nice because `'A'` and `'a'` appear, and `'B'` and `'b'` appear. However, `"abA"` is not because `'b'` appears, but `'B'` does not.
 *
 * Given a string `s`, return _the longest **substring** of `s` that is **nice**. If there are multiple, return the substring of the **earliest** occurrence. If there are none, return an empty string_.
 *
 * **Example 1:**
 *
 * **Input:** s = "YazaAay"
 *
 * **Output:** "aAa"
 *
 * **Explanation:** "aAa" is a nice string because 'A/a' is the only letter of the alphabet in s, and both 'A' and 'a' appear. "aAa" is the longest nice substring.
 *
 * **Example 2:**
 *
 * **Input:** s = "Bb"
 *
 * **Output:** "Bb"
 *
 * **Explanation:** "Bb" is a nice string because both 'B' and 'b' appear. The whole string is a substring.
 *
 * **Example 3:**
 *
 * **Input:** s = "c"
 *
 * **Output:** ""
 *
 * **Explanation:** There are no nice substrings.
 *
 * **Constraints:**
 *
 * *   `1 <= s.length <= 100`
 * *   `s` consists of uppercase and lowercase English letters.
**/
public class Solution {
    public String longestNiceSubstring(String s) {
        int index = isNotNiceString(s);
        if (index == -1) {
            return s;
        }
        String left = longestNiceSubstring(s.substring(0, index));
        String right = longestNiceSubstring(s.substring(index + 1));
        return left.length() >= right.length() ? left : right;
    }

    private int isNotNiceString(String s) {
        Set set = new HashSet<>();
        for (char c : s.toCharArray()) {
            set.add(c);
        }
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (!set.contains(Character.toLowerCase(c))
                    || !set.contains(Character.toUpperCase(c))) {
                return i;
            }
        }
        return -1;
    }
}