/in/foo.c:19:14: error: expected ';', ',' or ')' before '&' token
 void swap(int& a, int& b) {
              ^
/in/foo.c: In function 'isLuo':
/in/foo.c:26:24: warning: implicit declaration of function 'swap' [-Wimplicit-function-declaration]
  if (black_1 < black_2)swap(black_1, black_2);
                        ^~~~