/in/foo.c: In function 'main':
/in/foo.c:26:12: error: subscripted value is neither array nor pointer nor vector
    if(check[s1[i]] != s1[len-i-1]) b = 0;
            ^
/in/foo.c:17:9: warning: variable 'a' set but not used [-Wunused-but-set-variable]
     int a = 1, b = 1, len = 0;
         ^