/in/foo.c:4:1: error: unknown type name 'using'
 using namespace std;
 ^~~~~
/in/foo.c:4:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
 using namespace std;
                 ^~~
/in/foo.c: In function 'main':
/in/foo.c:15:11: warning: implicit declaration of function 'next_permutation' [-Wimplicit-function-declaration]
     while(next_permutation(a,a+9));
           ^~~~~~~~~~~~~~~~