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class Solution {
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  public String frequencySort(String s) {
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    int len = s.length();
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    HashMap<Integer, HashSet<Character>> map = new HashMap();
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    HashMap<Character, Integer> freqMap = new HashMap();
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    for (int idx = 0; idx < len; idx++) {
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      char ch = s.charAt(idx);
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      freqMap.put(ch, freqMap.getOrDefault(ch, 0) + 1);
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    }
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    int maxFreq = 0, minFreq = s.length();
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    for (char ch : freqMap.keySet()) {
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      HashSet<Character> set = map.getOrDefault(freqMap.get(ch), new HashSet());
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      set.add(ch);
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      map.put(freqMap.get(ch), set);
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      maxFreq = Math.max(maxFreq, freqMap.get(ch));
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      minFreq = Math.min(minFreq, freqMap.get(ch));
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    }
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    StringBuilder ansStr = new StringBuilder();
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    for (int freq = maxFreq; freq >= minFreq; freq--) {
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      if (map.containsKey(freq)) {
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        HashSet<Character> set = map.get(freq);
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        for (char ch : set) {
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          int temp = freq;
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          while (temp > 0) {
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            ansStr.append(ch);
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            temp--;
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          }
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        }
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      }
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    }
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    return ansStr.toString();
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  }
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}