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 Kotlin (1.0.0) (judge verdict: Accepted)

@file:Suppress("DuplicatedCode")

import java.io.IOException
import java.io.InputStream
import java.io.PrintWriter
import java.util.*

fun main(args: Array<String>) {
    PrintWriter(System.out).use { out ->
        D.solve(D.FastScanner(), out)
        out.flush()
    }
}

private fun D.solve(sc: D.FastScanner, out: PrintWriter) {
    val n = sc.nextInt()
    val map = HashMap<Char, HashMap<Char, HashSet<Char>>>()
    sc.toCharIterator().forEach { chr ->
        map.forEach { ee1 ->
            val m2 = ee1.value
            m2.forEach { ee2 ->
                ee2.value.add(chr)
            }

            if (chr !in m2) {
                m2[chr] = HashSet<Char>()
            }
        }

        if (chr !in map) {
            map[chr] = HashMap<Char, HashSet<Char>>()
        }
    }

    val size = map.map { e1 ->
        e1.value.map { e2 ->
            e2.value.size
        }.sum()
    }.sum()

    println(size)
}

@Suppress("MemberVisibilityCanBePrivate")
private object D {
    private const val zeroCode = '0'.toInt()
    private const val nineCode = '9'.toInt()

    class FastScanner {
        private val `in`: InputStream = System.`in`
        private val buffer = ByteArray(2048)
        private var ptr = 0
        private var buflen = 0

        private fun hasNextByte(): Boolean {
            if (ptr < buflen) {
                return true
            } else {
                ptr = 0
                try {
                    buflen = `in`.read(buffer)
                } catch (e: IOException) {
                    e.printStackTrace()
                }
                if (buflen <= 0) {
                    return false
                }
            }
            return true
        }

        private fun readByte(): Byte = if (hasNextByte()) buffer[ptr++] else -1

        private fun readByteAsInt(): Int = if (hasNextByte()) buffer[ptr++].toInt() else -1

        operator fun hasNext(): Boolean {
            while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++
            return hasNextByte()
        }

        operator fun next(): String {
            if (!hasNext()) throw NoSuchElementException()

            return buildString {
                var b = readByteAsInt()

                while (isPrintableChar(b)) {
                    appendCodePoint(b)
                    b = readByteAsInt()
                }
            }
        }

        fun toCharIterator(): CharIterator = object : CharIterator() {
            private var nextByte: Byte = readByte()

            override fun hasNext(): Boolean = isPrintableChar(nextByte)

            override fun nextChar(): Char {
                val res = nextByte
                nextByte = readByte()
                return res.toChar()
            }

        }

        fun nextLong(): Long {
            if (!hasNext()) throw NoSuchElementException()
            var n: Long = 0
            var minus = false
            var b = readByteAsInt()

            if (b == '-'.toInt()) {
                minus = true
                b = readByteAsInt()
            }

            if (b !in zeroCode..nineCode) {
                throw NumberFormatException()
            }

            while (true) {
                if (b in zeroCode..nineCode) {
                    n *= 10
                    n += b - zeroCode.toLong()
                } else if (b == -1 || !isPrintableChar(b)) {
                    return if (minus) -n else n
                } else {
                    throw NumberFormatException()
                }

                b = readByteAsInt()
            }
        }

        fun nextInt(): Int {
            val nl = nextLong()
            if (nl < Int.MIN_VALUE || nl > Int.MAX_VALUE) throw NumberFormatException()
            return nl.toInt()
        }

        fun nextDouble(): Double {
            return next().toDouble()
        }

        fun readIntArray(n: Int): IntArray {
            val arr = IntArray(n)

            for (i in 0 until n) {
                arr[i] = nextInt()
            }

            return arr
        }

        fun readBigInt(capacity: Int): IntArray {
            if (!hasNext()) throw NoSuchElementException()

            val arr = IntArray(capacity)
            var len = 0
            arr[0] = readByteAsInt() - zeroCode
            while (arr[len] in 0..9) {
                len++
                arr[len] = readByteAsInt() - zeroCode
            }

            return arr.sliceArray(0 until len)
        }

        companion object {
            private inline fun isPrintableChar(c: Int): Boolean {
                return c in 33..126
            }

            private inline fun isPrintableChar(c: Byte): Boolean {
                return c in 33..126
            }
        }
    }
}