LeetCode38|根据字符出现频率排序

输入:
"tree"

输出:
"eert"

解释:
'e'出现两次，'r'和't'都只出现一次。
因此'e'必须出现在'r'和't'之前。此外，"eetr"也是一个有效的答案。

import java.util.*;
import java.util.stream.Collectors;

public class FrequencySortTest {
    public static void main(String[] args) {
        String str = "tree";
        String frequencySort = frequencySort(str);
        System.out.println("frequencySort = " + frequencySort);
    }

    public static String frequencySort(String s) {
        if (s == null || s.length() == 0) {
            return s;
        }
        char[] chars = s.toCharArray();
        int length = chars.length;
        HashMap<Character, Integer> hashMap = new HashMap<>(length);
        for (Character c : chars) {
            hashMap.put(c, hashMap.getOrDefault(c, 0) + 1);
        }
        StringBuilder stringBuilder = new StringBuilder(length);
        List<Map.Entry<Character, Integer>> entryList = hashMap.entrySet().stream().
                sorted((o1, o2) -> o2.getValue().compareTo(o1.getValue())).collect(Collectors.toList());
        LinkedHashMap<Character, Integer> map = entryList.stream().collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));
        map.forEach((key, value) -> {
            while (value > 0) {
                stringBuilder.append(key);
                value -= 1;
            }
        });
        return stringBuilder.toString();
    }
}