Greatest English Letter in Upper and Lower Case LeetCode solution in Java

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class Solution {
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  public String greatestLetter(String s) {
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    Set<Character> set = new HashSet<>();
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    for (char ch : s.toCharArray()) set.add(ch);
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    for (char ch = 'Z'; ch >= 'A'; ch--)
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      if (set.contains(ch) && set.contains((char) ('a' + (ch - 'A')))) return "" + ch;
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    return "";
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  }
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}