/in/foo.cc: In function 'int main()':
/in/foo.cc:27:18: warning: operation on 'k' may be undefined [-Wsequence-point]
   if(s[i]-'0') f[++k]= 2*f[k-1]%MOD + f_pow(1ll*4,1ll*(k-1)) % MOD;
                  ^~~
/in/foo.cc:27:18: warning: operation on 'k' may be undefined [-Wsequence-point]
/in/foo.cc:28:10: warning: operation on 'k' may be undefined [-Wsequence-point]
   else f[++k]= 2*f[k-1]%MOD;
          ^~~