Explain the difference amongst the following 3 functions:

void swap1(int x, int y) {

       int temp = x;

       x = y;

       y = temp;

}

void swap3(int *x, int *y) {

       int temp = *x;

       *x = *y;

       *y = temp;

}

void swap2(int &x; int &y) {

        int temp = x;

        x = y;

        y = temp;

}