sets

Sets also come in read-only and mutable flavors. As its name implies, sets contain unique values.

val planets = setOf("Mercury", "Venus", "Earth", "Earth")
// ["Mercury", "Venus", "Earth"]

planets.contains("Pluto") // false
"Earth" in planets // true

planets[2] // Error, as sets don't index its contents
planets.elementAt(2) // O(n) operation

val heroes = mutableSetOf("Batman", "Superman")
heroes.add("Wonder Woman")

val listOfHeroes = heroes.toList() // converting a `set` to a `list`
listOfHeroes.toSet() // converting a `list` to a `set`