Challenge 2: swap(int *a, int *b) — Possible Solution
====================================================================

#include <stdio.h>

void swap(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

int main() {
    int x = 1;
    int y = 2;

    printf("Before: x=%d y=%d\n", x, y);
    swap(&x, &y);
    printf("After:  x=%d y=%d\n", x, y);

    return 0;
}

Output:

    Before: x=1 y=2
    After:  x=2 y=1

WHY THIS WORKS AS AN ANSWER
------------------------------
swap receives pointers to x and y (their addresses via &x/&y, not
copies of their values), so *a and *b inside the function refer
directly to main's own variables -- the temp-based swap genuinely
exchanges their contents. This is exactly Chapter 5's own
pass-by-value limitation solved: swapping two ints by value alone is
impossible in C (the function would only swap its own local copies),
which is precisely why pointers, introduced in this chapter, exist.