最大字典序子序列

从右往左遍历，遇到小的就删：

import java.util.*;
public class Main{
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        StringBuilder sb = new StringBuilder(sc.nextLine());
        char last = sb.charAt(sb.length() - 1);
        int i = sb.length() - 2;
        while(i >= 0){
            if(sb.charAt(i) >= last){
                last = sb.charAt(i);
                i--;
            }else{
                sb.deleteCharAt(i);
            }
        }
        System.out.println(sb);
    }
}