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.

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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.

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Constraints:

• 1 <= s.length <= 100

• s?consists of uppercase and lowercase English letters.‘

• 題目不難，就是寫的code長了一點，先是要判斷是否是nice string，用數(shù)組最快了，

• 然后遍歷整個字符串，條件是當j-i+1沒在hashmap中，因為如果在了，說明前面已經(jīng)有這么長的字符串滿足要求了，所以就不需要判斷的，如果是nice string，就把長度跟左右端的index放到map中，然后遍歷map，找到最長的，返回即可（如果map.size>1的情況下），map如果是空的就返回空字符串即可。

Runtime:?13 ms, faster than?30.49%?of?Java?online submissions for?Longest Nice Substring.

Memory Usage:?42.6 MB, less than?30.32%?of?Java?online submissions for?Longest Nice Substring.