/**
 * Complete the function to return the union of two lists using the OR operator and null as a placeholder.
 *
 * @param list1 The first input list.
 * @param list2 The second input list.
 * @return The union of the two input lists.
 */
fun unionOfLists(list1: List<Int>?, list2: List<Int>?): List<Int>? {

    if (list1 == null && list2 == null) {
        return null
    } else if (list1 == null) {
        return list2
    } else if (list2 == null) {
        return list1
    } else {
        return (list1.toSet() union list2.toSet()).toList()
    }
}
/**
 * Complete the function to return the union of two lists using the OR operator and null as a placeholder.
 *
 * @param list1 The first input list.
 * @param list2 The second input list.
 * @return The union of the two input lists.
 */
fun unionOfLists(list1: List<Int>?, list2: List<Int>?): List<Int>? {
/**
 * Complete the function to return the union of two lists using the OR operator and null as a placeholder.
 *
 * @param list1 The first input list.
 * @param list2 The second input list.
 * @return The union of the two input lists.
 */
fun unionOfLists(list1: List<Int>?, list2: List<Int>?): List<Int>? {

    if (list1 == null && list2 == null) {
        return null
    } else if (list1 == null) {
        return list2
    } else if (list2 == null) {
        return list1
    } else {
        return (list1.toSet() union list2.toSet()).toList()
    }
}