Sort Characters by Frequency

Given a string, sort it in decreasing order based on the frequency of characters.

Example 1:

Input:
"tree"

Output:
"eert"

Explanation:
'e' appears twice while 'r' and 't' both appear once.
So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.

Example 2:

Input:
"cccaaa"

Output:
"cccaaa"

Explanation:
Both 'c' and 'a' appear three times, so "aaaccc" is also a valid answer.
Note that "cacaca" is incorrect, as the same characters must be together.

Example 3:

Input:
"Aabb"

Output:
"bbAa"

Explanation:
"bbaA" is also a valid answer, but "Aabb" is incorrect.
Note that 'A' and 'a' are treated as two different characters.

public class Solution {
    public String frequencySort(String s) {
        HashMap<Character, Integer> map = new HashMap<Character, Integer>();
        for (int i = 0; i < s.length(); i++) {
            if (map.containsKey(s.charAt(i)))
                map.put(s.charAt(i), map.get(s.charAt(i)) + 1);
            else
                map.put(s.charAt(i), 1);
        }
        
        PriorityQueue< Map.Entry<Character, Integer> > pq = new PriorityQueue<>(
            new Comparator<Map.Entry<Character, Integer> >() {
                public int compare (Map.Entry<Character, Integer> a, Map.Entry<Character, Integer> b) {
                    return b.getValue() - a.getValue();
                }
            }
        );
        
        pq.addAll(map.entrySet());
        StringBuilder builder = new StringBuilder();
        while (!pq.isEmpty()) {
            Map.Entry entry = pq.poll();
            int frequency = (int) entry.getValue();
            for (int i = 0; i < frequency; i++) {
                builder.append(entry.getKey());
            }
        }
        return builder.toString();
    }
}