/in/foo.c: In function 'main':
/in/foo.c:24:5: error: 'cin' undeclared (first use in this function)
     cin >> n1 >> n2 >> n3;
     ^~~
/in/foo.c:24:5: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:37:9: error: 'cout' undeclared (first use in this function)
         cout << merged12[i] << " ";
         ^~~~
/in/foo.c:39:13: error: 'endl' undeclared (first use in this function)
     cout << endl;
             ^~~~