18. The method to swap array elements works because, unlike integers, arrays are objects and use reference semantics.
19.
2 [0, 0, 1, 0]
1 [0, 0, 1, 0]
3 [0, 0, 1, 1]
2 [0, 0, 1, 1]
20.
2 [0, 1]
1 [0, 1]
1 [1, 2]
0 [1, 2]
21.
public static void swapPairs(int[] list) {
    for (int i = 0; i < list.length - 1; i += 2) {
        swap(list, i, i + 1);
    }
}