451_Sort_Characters_By_Frequency.java

classSolution {
publicStringfrequencySort(Strings) {
if (s == null || s.length() == 0) {
returns;
 }

HashMap<Character, Integer> hm = newHashMap<>();
PriorityQueue<Map.Entry<Character, Integer>> pq = newPriorityQueue<>((a, b) -> b.getValue() - a.getValue());
StringBuildersb = newStringBuilder();

for (charc : s.toCharArray()) {
hm.put(c, hm.getOrDefault(c, 0) + 1);
 }

pq.addAll(hm.entrySet());

while (!pq.isEmpty()) {
Map.Entry<Character, Integer> entry = pq.poll();

for (inti = 0; i < entry.getValue(); i++) {
sb.append(entry.getKey());
 }
 }

returnsb.toString();
 }
}