Set

Set is the subclass of Iterable.

val fruits = Set("apple", "banana")
val s = (1 until 6).toSet

s.map(_ + 2)
fruits.filter(_.startWith("app"))

1. Set is unordered.

2. Set's elements are unique.

3. contains function is very useful in Set

   s.contains(1) // true
   

Exercise

def meetQueen(col: Int, intercept: Int, queens: List[Int]): Boolean = queens match {
  case Nil => false
  case q :: others => {
    q == col || (q - col).abs == intercept || meetQueen(col, intercept + 1, others)
  }
}

def queens(n: Int) = {
  def playQueens(k: Int): Set[List[Int]] = {
    if (k == 0) Set(List())
    else {
      for (
        queens <- playQueens(k - 1);
        col <- 0 until n
        if !meetQueen(col, 1, queens)
      ) yield col :: queens
    }
  }
  playQueens(n)
}