Problem:
Score: 400 points

Problem Statement

AtCoder Inc. has decided to lock the door of its office with a 3-digit PIN code.

The company has an N-digit lucky number, S. Takahashi, the president, will erase N-3 digits from S and concatenate the remaining 3 digits without changing the order to set the PIN code.

How many different PIN codes can he set this way?

Both the lucky number and the PIN code may begin with a 0.

Constraints

4 \leq N \leq 30000

S is a string of length N consisting of digits.

Input

Input is given from Standard Input in the following format:

N
S

Output

Print the number of different PIN codes Takahashi can set.

Sample Input 1

4
0224

Sample Output 1

3

Takahashi has the following options:

Erase the first digit of S and set 224.

Erase the second digit of S and set 024.

Erase the third digit of S and set 024.

Erase the fourth digit of S and set 022.

Thus, he can set three different PIN codes: 022, 024, and 224.

Sample Input 2

6
123123

Sample Output 2

17

Sample Input 3

19
3141592653589793238

Sample Output 3

329

Solution in Scala (2.13.1) (judge verdict: Accepted)

object Main {

  def main(args: Array[String]): Unit = {
    val sc = new java.util.Scanner(System.in)

    val N = sc.nextInt(); sc.nextLine()
    val S = sc.nextLine().toList
    // 3桁の暗証番号, N桁のラッキーナンバーS, SからN - 3桁を消して, 残りの3桁を 左から読んだものを暗証番号として設定
    // 設定されうる暗証番号は何種類?
    // 残す桁を一つづつ選ぶ場合だと, N^3 なので, 間に合わない.
    // 最大で, 9 ^ 3種類しかない, というのはわかっている
    // 逆に, 9 ^ 3種類の中からSで満たすものがあるか？を調べた方が良さそう
    // 9 ^ 3通りの組み合わせで, Nについて走査すればいい
    def check(a: Int, b: Int, c: Int): Boolean = {
      def recursive(s: List[Char], rest: List[Char]): Boolean =
        rest match {
          case Nil => true
          case h :: t =>
            s match {
              case Nil => false
              case hh :: tt =>
                if (h == hh) recursive(tt, t)
                else recursive(tt, rest)
            }
        }
      recursive(S, List(a, b, c).map(i => (i + '0').toChar))
    }

    val r = (for {
      a <- 0 to 9
      b <- 0 to 9
      c <- 0 to 9
      if check(a, b, c)
    } yield (a, b, c)).size



    println(r)
  }

}