/in/foo.cc: In function 'bool check()':
/in/foo.cc:26:23: warning: array subscript has type 'char' [-Wchar-subscripts]
    if (head) v[s[head]] = 0, head --;
                       ^
/in/foo.cc:29:14: warning: array subscript has type 'char' [-Wchar-subscripts]
    if (v[a[i]]) return false;
              ^
/in/foo.cc:30:10: warning: array subscript has type 'char' [-Wchar-subscripts]
    v[a[i]] = 1;
          ^