/in/foo.cc: In function 'int main()':
/in/foo.cc:39:16: warning: array subscript has type 'char' [-Wchar-subscripts]
    if(F[s[head]] == 1 && F[s[tail]] == 1 && s[head] == s[tail])  head++,tail--;
                ^
/in/foo.cc:39:35: warning: array subscript has type 'char' [-Wchar-subscripts]
    if(F[s[head]] == 1 && F[s[tail]] == 1 && s[head] == s[tail])  head++,tail--;
                                   ^