public String solve (String s, String t) {
        if (s==null || t==null) return null;
        // write code here
        int i = s.length()-1;
        int j = t.length()-1;
        int jw=0;
        Stack<Integer> stack=new Stack<>();
        while (i>=0 && j>=0){
            int si = Integer.parseInt(s.charAt(i)+"");
            int sj = Integer.parseInt(t.charAt(j)+"");
            int zh = si+sj+jw;
            jw = zh/10;
            int res = zh%10;
            stack.add(res);
            i--;
            j--;
        }
        //i s stack
        jw = zz(i,s,stack,jw);
        jw = zz(j,t,stack,jw);
        if (jw>0) stack.add(jw);
        StringBuilder sb = new StringBuilder();
        while (stack.size()>0){
            sb.append(stack.pop());
        }
        return sb.toString();
    }
    public int  zz(int i,String s,Stack<Integer> stack,int jw){
        while (i>=0){
            int si = Integer.parseInt(s.charAt(i)+"");
            int zh = si+jw;
            stack.add(zh%10);
            jw = zh/10;
            i--;
        }
        return jw;
    }