题解 | #字符串排序#

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        while (in.hasNextLine()) {
            String str = in.nextLine();

            // 收集所有字母
            List<Character> list = new ArrayList<>();
            final char[] chars = str.toCharArray();
            for (char ch : chars) {
                if (isLetter(ch)) {
                    list.add(ch);
                }
            }

            // 对字母排序
            list.sort(Comparator.comparingInt(Character::toLowerCase));

            // 遍历，如果是字母，则从集合中取；如果不是字母，则不变
            int i = 0;
            StringBuilder sb = new StringBuilder();
            for (char ch : chars) {
                if (isLetter(ch)) {
                    sb.append(list.get(i));
                    i++;
                } else {
                    sb.append(ch);
                }
            }
            // 输出
            System.out.println(sb.toString());
        }
    }

    /**
     * 判断是否是字母
     */
    private static boolean isLetter(char ch) {
        return (ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch <= 'z');
    }
}