451. Sort Characters By Frequency

先统计次数，再按次数构造新的string。思路没什么难，写起来还有点麻烦。用了priorityqueue。。

    class Wrapper {
        public char c;
        public int freq;

        public Wrapper(char c, int freq) {
            this.c = c;
            this.freq = freq;
        }
    }

    public String frequencySort(String s) {
        if (s==null || s.length() == 0) return "";
        Map<Character, Integer> map = new HashMap<>();
        Set<Character> set = new HashSet<>();
        for (int i = 0; i < s.length(); i++) {
            int count = map.getOrDefault(s.charAt(i), 0);
            map.put(s.charAt(i), ++count);
            set.add(s.charAt(i));
        }
        PriorityQueue<Wrapper> pq = new PriorityQueue<>(new Comparator<Wrapper>() {
            @Override
            public int compare(Wrapper o1, Wrapper o2) {
                return o1.freq - o2.freq;
            }
        });
        for (char c : set) {
            pq.offer(new Wrapper(c, map.get(c)));
        }
        StringBuilder sb = new StringBuilder();
        while (!pq.isEmpty()) {
            Wrapper w = pq.poll();
            for (int i = 0; i < w.freq; i++) {
                sb.insert(0, w.c);
            }
        }
        return sb.toString();
    }

因为用了PriorityQueue所以复杂度是O(nlogn)。
看了别人的答案，有个是O(n)的，用了数组的index表示出现的次数。而且，我上面用了hashset，因为我不太会hashmap的遍历，下面的代码也展示了hashmap的遍历方法, for(char c : map.keySet())：

public String frequencySort(String s) {
    if (s == null) {
        return null;
    }
    Map<Character, Integer> map = new HashMap();
    char[] charArray = s.toCharArray();
    int max = 0;
    for (Character c : charArray) {
        if (!map.containsKey(c)) {
            map.put(c, 0);
        }
        map.put(c, map.get(c) + 1);
        max = Math.max(max, map.get(c));
    }

    List<Character>[] array = buildArray(map, max);

    return buildString(array);
}

private List<Character>[] buildArray(Map<Character, Integer> map, int maxCount) {
    List<Character>[] array = new List[maxCount + 1];
    for (Character c : map.keySet()) {
        int count = map.get(c);
        if (array[count] == null) {
            array[count] = new ArrayList();
        }
        array[count].add(c);
    }
    return array;
}

private String buildString(List<Character>[] array) {
    StringBuilder sb = new StringBuilder();
    for (int i = array.length - 1; i > 0; i--) {
        List<Character> list = array[i];
        if (list != null) {
            for (Character c : list) {
                for (int j = 0; j < i; j++) {
                    sb.append(c);
                }
            }
        }
    }
    return sb.toString();
}