What's the problem with this code in Secret Messages?

Question

import java.util.Scanner;
public class Program
{
public static void main(String[] args) {
Scanner sc=new Scanner(System.in);
String str=sc.nextLine();
String str1=str.toLowerCase();
String s1="abcdefghijklmnopqrstuvwxyz";
String s2="zyxwvutsrqponmlkjihgfedcba";
String[]arr1=s1.split("");
String[]arr2=s2.split("");
for(int i=0;i<arr1.length-1;i++){
str1=str1.replaceAll(arr1[i],arr2[i]);
}
System.out.println(str1);
}
}

Answer 1

The problem is you will switch b for y, and then you will switch y to b, but the previous b is no more a b...
Example:
abcz -> zycz-> zbxz -> zbcz -> abca
Your input is not going to be abca but zbcz because you do your loop <arr1.length-1 so you will skip the last position (the z)
(Edit)
One solution could be:
loop only to half of the arr1
replaceAll(arr1[i], '*');
replaceAll(arr2[i],arr1[i]);
replaceAll('*',arr2[i]);

Answer 2

Input :
aaa => zzz => aaa
result