Challenge 1: List, Set, and Map Basics — Solution

fun main() {
    val cities = listOf("London", "Paris", "Berlin", "Paris", "Madrid")
    val uniqueCities = cities.toSet()

    println(cities)
    println(uniqueCities)

    val populations = mapOf("London" to 9, "Paris" to 2, "Berlin" to 4)
    println("Paris population: ${populations["Paris"]} million")
}

/*
Output:
[London, Paris, Berlin, Paris, Madrid]
[London, Paris, Berlin, Madrid]
Paris population: 2 million

Notes:
- cities is a List, so it keeps both "Paris" entries in their original
  order.
- .toSet() converts the List to a Set, which drops the duplicate "Paris"
  automatically — sets never contain repeated elements.
- populations["Paris"] performs a key lookup on the Map, returning the
  Int value (2) associated with that key.
*/