样例的答案都是正确的,思路也没问题

import java.io.*;

public class Main
{
    static String str;
    static int[] index;
    static int pointer=0;

    public static void main(String[] args) throws IOException
    {
        StreamTokenizer st=new StreamTokenizer(new BufferedReader(new InputStreamReader(System.in)));
        BufferedReader bf=new BufferedReader(new InputStreamReader(System.in));
        PrintWriter pw=new PrintWriter(new OutputStreamWriter(System.out));
        str=bf.readLine();
        st.nextToken();
        int k=str.length()-(int)st.nval;
        index=new int[k];
        find(0);
        for(int i=0,flag=0;i<k;i++)
        {
            if(str.charAt(index[i])=='0'&&flag==0&&i<k-1)
                continue;
            pw.print(str.charAt(index[i]));
            flag=1;
        }
        pw.flush();
    }

    public static void find(int start)
    {
        if(pointer==index.length)
            return;
        int flag=0;
        for(char i='0';i<='9'&&flag==0;i++)
        {
            for(int j=start;j<str.length();j++)
            {
                if(str.charAt(j)==i)
                {
                    if(str.length()-j<index.length-pointer)
                        break;
                    else
                    {
                        index[pointer++]=j;
                        find(j+1);
                        flag=1;
                        break;
                    }
                }
            }
        }
    }
}