week6 451. Sort Characters By Frequency

public class Solution {
    public String frequencySort(String s){
        char[] array = s.toCharArray();
        HashMap<Character, Integer> map = new HashMap<Character, Integer>();
        int max = 1;
        for (int i = 0; i < array.length; i++) {
            Integer temp = map.get(array[i]);
            if (temp == null) {
                map.put(array[i], 1);
            } else {
                map.put(array[i], temp + 1);
                max = Math.max(max, temp + 1);
            }
        }
        List<Character>[] list = new List[max + 1];
        for (Character c : map.keySet()) {
            int num = map.get(c);
            if (list[num] == null) {
                list[num] = new ArrayList();
            }
            list[num].add(c);
        }
        StringBuilder sb = new StringBuilder();
        for (int i = list.length - 1; i >= 0; i--) {
            List<Character> subList = list[i];
            if (subList != null) {
                for (Character c : subList) {
                    for (int j = 0; j < i; j++) {
                        sb.append(c);
                    }
                }
            }
        }
        return sb.toString();
    }
}