451. Sort Characters By Frequency 

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class Solution {
    public String frequencySort(String s) {
        Map<Character, Integer> charToFrequencyMap = new HashMap<>();
        int maxFrequency = 0;
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            int frequency = charToFrequencyMap.getOrDefault(c, 0) + 1;
            maxFrequency = Math.max(frequency, maxFrequency);
            charToFrequencyMap.put(c, frequency);
        }

        StringBuilder[] sbArray = new StringBuilder[maxFrequency + 1];

        for (Map.Entry<Character, Integer> entry : charToFrequencyMap.entrySet()) {
            char c = entry.getKey();
            int frequency = entry.getValue();

            StringBuilder sb = sbArray[frequency];
            if (sb == null) {
                sbArray[frequency] = sb = new StringBuilder();
            }

            sb.append(c);
        }

        StringBuilder resBuilder = new StringBuilder();
        for (int i = maxFrequency; i > 0; i--) {
            StringBuilder sb = sbArray[i];
            if (sb != null) {
                for (int j = 0; j < sb.length(); j++) {
                    for (int k = 0; k < i; k++) {
                        resBuilder.append(sb.charAt(j));
                    }
                }
            }
        }

        return resBuilder.toString();
    }
}

References

451. Sort Characters By Frequency