# 1763. Longest Nice Substring

https://leetcode.com/problems/longest-nice-substring

## Description

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.
```

**Example 4:**

```
**Input:** s = "dDzeE"
**Output:** "dD"
**Explanation:** Both "dD" and "eE" are the longest nice substrings.
As there are multiple longest nice substrings, return "dD" since it occurs earlier.
```

**Constraints:**

* `1 <= s.length <= 100`
* `s` consists of uppercase and lowercase English letters.

## ac

```java
```
