451. 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.
题目含义：按字母出现次数的降序重排字符串

    public String frequencySort(String s) {
        Map<Character, Integer> map = new HashMap<>();
        for (int i = 0; i < s.length(); i++) {
            map.put(s.charAt(i), map.getOrDefault(s.charAt(i), 0)+1);
        }
        PriorityQueue<Map.Entry<Character, Integer>> pq = new PriorityQueue<>(
                new Comparator<Map.Entry<Character, Integer>>() {
                    @Override
                    public int compare(Map.Entry<Character, Integer> a, Map.Entry<Character, Integer> b) {
                        return b.getValue() - a.getValue();
                    }
                }
        );
        pq.addAll(map.entrySet());
        StringBuilder sb = new StringBuilder();
        while (!pq.isEmpty()) {
            Map.Entry<Character, Integer> e = pq.poll();
            for (int i = 0; i < e.getValue(); i++) {
                sb.append(e.getKey());
            }
        }
        return sb.toString();        
    }