题解 | #一种字符串和数字的对应关系#

import java.util.Scanner;

public class Main {
    public static String getString(char[]chs,int n){
        if(chs==null||chs.length==0||n<1){
            return "";
        }
        int cur=1;
        int base= chs.length;
        int len=0;
        while (n>=cur){
            len++;
            n-=cur;
            cur*=base;
        }
        char[] res=new char[len];
        int index=0;
        int nCur=0;
        do{
            cur/=base;
            nCur=n/cur;
            res[index++]=getK(chs,nCur+1);
            n%=cur;
        }while (index!=res.length);
        return String.valueOf(res);
    }

    //得到字符串数组的第k-1个字符
    public static char getK(char[]chs,int k){
        if(k<1||k> chs.length){
            return 0;
        }
        return chs[k-1];
    }

    /*字符串数组得到数字
    * */
    public static int getNum(char[]chs,String str){
        if(chs==null||chs.length==0){
            return 0;
        }
        char[] strc=str.toCharArray();
        int base= chs.length;   //进制
        int cur=1;
        int res=0;
        for (int i = strc.length-1; i !=-1 ; i--) {
            res+=getNth(chs,strc[i])*cur;
            cur*=base;
        }
        return res;

    }
    //由字符得到对应数字
    public static int getNth(char[] chs,char ch){
        int res=-1;
        for (int i = 0; i < chs.length; i++) {
            if(chs[i]==ch){
                res=i+1;
                break;
            }
        }
        return res;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String[] stt1=in.nextLine().split(" ");
        int opt = Integer.parseInt(stt1[0]);
        int len = Integer.parseInt(stt1[1]);
        String str=in.nextLine();
        char[] chs=str.toCharArray();
        if(opt==1){
            int n=in.nextInt();
            System.out.println(getString(chs,n));
        }else {
            String str_in=in.nextLine();
            System.out.println(getNum(chs,str_in));
        }


    }    
}