22-SortCharactersByFrequency.java

class Solution {
    public String frequencySort(String s) {
        int len = s.length();
        if (len == 0) return "";
        int[] map = new int[128];
        for (char c : s.toCharArray()) {
            map[c - '\u0000']++;
        }
        // System.out.println(Arrays.toString(map));
        PriorityQueue<Pair<Character, Integer>> pq = new PriorityQueue<>(new SortByFreq());
        for (int i = 0; i < 128; i++) {
            if (map[i] != 0) {
                pq.add(new Pair((char) i, map[i]));
            }
        }

        StringBuilder ans = new StringBuilder();
        while (!pq.isEmpty()) {
            Pair<Character, Integer> p = pq.poll();
            for (int i = 0; i < p.getValue(); i++) {
                ans.append(p.getKey());
            }
        }
        return ans.toString();
    }

    class SortByFreq implements Comparator<Pair<Character, Integer>> {

        public int compare(Pair<Character, Integer> p1, Pair<Character, Integer> p2) {
            return p2.getValue() - p1.getValue();
        }
    }
}