紫皮 困难的串(Java) P195

package basic;
import java.util.Scanner;

public class kunnandechuan {
    static int count;
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int L = sc.nextInt();
        String s = "";
        dfs(n,L,s);
    }
    public static void dfs(int n,int L,String s){
        for(char i = 'A';i < 'A' + L;i++){
            if (isHard(s,i)){
                String ss = s + i;
                count++;//计数,字典序第n小时打印
                if(count == n){
                    System.out.println(ss);
                    System.exit(0);
                }
                dfs(n,L,ss);
            }
        }
    }
    public static boolean isHard(String s,char i){
        int len = 0;
        for(int j = s.length()-1;j>=0;j-=2) {
            String s1 = s.substring(j,j+len+1);
            String s2 = s.substring(j+len+1)+i;
            len++;
            if(s1.equals(s2)) {
                return false;
            }
        }
        return true;
    }
}