A中插一个B，几种插法是回文？

用StringBuilder的insert遍历插入，用reverse和equals判断回文：

import java.util.*;
public class Main {
    public static void main(String[] args) throws Exception{
        Scanner sc = new Scanner(System.in);
        StringBuilder A = new StringBuilder(sc.nextLine());
        StringBuilder B = new StringBuilder(sc.nextLine());
        int r = 0;
        for(int i = 0; i <= A.length(); ++i){
            StringBuilder temp = new StringBuilder(A);
            temp.insert(i, B);
            if(f(temp)) r++;
        }
        System.out.println(r);
    }
    static boolean f(StringBuilder s){    //判断回文串
        String s1 = s.toString();
        StringBuilder s2 = s.reverse();
        return s1.equals(s2.toString());
    }
}