C++ pass by reference when using pointer

[Lei]

compare following results:

void swap (int *p1, int *p2) {

int tmp = *p1;

*p1=*p2;

*p2=tmp;

}

n1 = 10;

n2 = 20;

swap(n1, n2);

swap(&n1, &n2);

you will get the same results, why?