Maximum Number of Occurrences of a Substring LeetCode solution in Java

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class Solution {
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  public int maxFreq(String s, int maxLetters, int minSize, int maxSize) {
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    Map<String, Integer> count = new HashMap<>();
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    int ans = 0;
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    int n = s.length();
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    for (int i = 0; i + minSize - 1 < n; i++) {
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      String y = s.substring(i, i + minSize);
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      count.put(y, count.getOrDefault(y, 0) + 1);
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      int unique = uniqueCharactersInString(y);
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      if (unique <= maxLetters) {
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        ans = Math.max(ans, count.get(y));
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      }
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    }
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    // System.out.println(count);
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    return ans;
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  }
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  int uniqueCharactersInString(String y) {
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    Set<Character> hs = new HashSet<>();
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    for (int j = 0; j < y.length(); j++) hs.add(y.charAt(j));
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    return hs.size();
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  }
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}