Challenge 3: flatMap — Solution

fun main() {
    val orders = listOf(
        listOf("Apple", "Bread"),
        listOf("Milk"),
        listOf("Eggs", "Cheese", "Butter")
    )

    val allItems = orders.flatMap { it }

    println(allItems)
    println("Total items ordered: ${allItems.size}")
}

/*
Output:
[Apple, Bread, Milk, Eggs, Cheese, Butter]
Total items ordered: 6

Notes:
- orders is a List<List<String>> — three inner lists, one per customer.
- flatMap { it } maps each inner list to itself, then flattens all three
  into a single List<String> with six elements total.
- Using plain map { it } instead would have kept the nesting, producing
  [[Apple, Bread], [Milk], [Eggs, Cheese, Butter]] — a list of three lists,
  not one flat list of six items.
- .size on the flattened list gives the total item count across every
  order in a single call.
*/