rev(rev(x) + rev(y))

用StringBuilder的reverse方法，和Integer的parseInt方法：

import java.util.*;
public class Main {
    public static void main(String[] args) throws Exception{
        Scanner sc = new Scanner(System.in);
        int x = sc.nextInt(), y = sc.nextInt();
        StringBuilder s1 = new StringBuilder(Integer.toString(x)).reverse();
        StringBuilder s2 = new StringBuilder(Integer.toString(y)).reverse();
        int r = Integer.parseInt(s1.toString()) + Integer.parseInt(s2.toString());
        StringBuilder ans = new StringBuilder(Integer.toString(r)).reverse();
        System.out.println(Integer.parseInt(ans.toString()));
    }
}