Answer 1

ans is already String then why this line

String p=Character.toString(ans);

You can't compare two String with == assignment. To compare String use equals method like this

a.equals(b)

Answer 2

now:

ans=""; // added, always reset ans for new token test
for(int j=l-1; j>=0; j--)
{
// ans = ans+Character.toString( s.charAt(j) );
ans += a.charAt(j); // check individual token a, instead whole input string s
}