对称数字

只有形如1000这种数需要变成999，其它都是把前半段反过来拷到后半段：

import java.util.*;
public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        String s = sc.nextLine();
        if(s.length() <= 1){
            System.out.println(s);
            return;
        }
        if(f(s)){    //只有形如1000这种数需要变成999
            String r = "";
            while(r.length() < s.length() - 1) r += "9";
            System.out.println(r);
            return;
        }
        StringBuilder str = new StringBuilder(s);
        StringBuilder sb = new StringBuilder(s).reverse();
        int n = s.length();
        str.replace(n/2, n, sb.substring(n/2, n));//把前半段反过来拷到后半段
        System.out.println(str);
    }
    static boolean f(String s){    //判断是不是形如1000这种数
        boolean flag = true;
        if(s.charAt(0) != '1') flag = false;
        if(s.charAt(0) == '1'){
            for(int i = 1; i < s.length(); ++i){
                if(s.charAt(i) != '0'){
                    flag = false;
                    break;
                }
            }
        }
        return flag;
    }
}