题解 | #字符统计#

import java.util.*;
import java.util.regex.Pattern;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String reg = "[0-9a-z]+";
        String a = in.nextLine();
        if (!Pattern.matches(reg, a)) {
            System.out.println("输入不合规");
            return;
        }
	  
        // 首先将数据封装到treeMap中，并默认以ASCII码进行升序排序
        Map<Character, Integer> map = new TreeMap<>();
        for (char c : a.toCharArray()) {
            if(map.get(c)==null) {
                map.put(c, 1);
            } else {
                map.put(c, map.get(c)+1);
            }
        }
        
		// 然后再把map封装到List中，以map的value值进行排序
		List<Map.Entry<Character, Integer>> entryList = new ArrayList<>(map.entrySet());
        entryList.sort(new Comparator<Map.Entry<Character, Integer>>() {
            @Override
            public int compare(Map.Entry<Character, Integer> o1, Map.Entry<Character, Integer> o2) {
                return o2.getValue().compareTo(o1.getValue());
            }
        });
		// forEach匿名内部类遍历
        entryList.forEach(new Consumer<Map.Entry<Character, Integer>>() {
            @Override
            public void accept(Map.Entry<Character, Integer> characterIntegerEntry) {
                System.out.print(characterIntegerEntry.getKey());
            }
        });
    }
}