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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Text.RegularExpressions;
using static System.Console;

namespace AtCoder_1
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = CinT();
            string S = Cin();
            int ct = 0;
            for(int i = 0;i <= 999; i++)
            {
                string res = i.ToString().PadLeft(3, '0');
                bool y = true;
                int sti = 0;
                for(int g = 0;g < 3; g++)
                {
                    sti = S.IndexOf(res[g], sti) + 1;
                    if(sti <= 0)
                    {
                        y = false;
                        break;
                    }
                }
                if (y)
                {
                    ct++;
                }
            }
            Cout(ct);
            Cin();

        }

        static long Get(long a,long b, long n)
        {
            return a * n + b * ((long)Math.Log10(n) + 1);
        }

        static Random Rand = new Random();
        static string[] AlphabetTable = new string[] { "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z" };

        static int loop(int num, int max)
        {
            if (num > max)
            {
                return num % max;
            }
            else if (num < 0)
            {
                return max - (-num) % max;
            }
            else
            {
                return num;
            }
        }
        static long GCD(long a, long b)
        {
            if (a < b) { swap(ref a, ref b); }
            if (a % b == 0) { return b; }
            else { return GCD(b, a % b); }
        }

        static void swap<T>(ref T a, ref T b)
        {
            T t = a;
            a = b;
            b = t;
        }

        static readonly int MOD10E9 = (1000000000 + 7);
        static bool isInt(double d) { return (int)d == d; }
        static string Cin() { return ReadLine(); }
        static string[] CinAr() { return ReadLine().Split(' '); }
        static int CinT() { return S2i(Cin()); }
        static int[] CinTAr() { return CinAr().Select((x, i) => { return S2i(x); }).ToArray(); }
        static long CinL() { return S2L(Cin()); }
        static long[] CinLAr() { return CinAr().Select((x, i) => { return S2L(x); }).ToArray(); }
        static double CinD() { return S2D(Cin()); }
        static double[] CinDAr() { return CinAr().Select((x, i) => { return S2D(x); }).ToArray(); }
        static int S2i(string s) { return int.Parse(s); }
        static long S2L(string s) { return long.Parse(s); }
        static double S2D(string s) { return double.Parse(s); }
        static void Cout(object s) { WriteLine(s); }
        static long[] yakusuu(long n)
        {
            if (n == 1) { return new long[] { 1 }; }
            List<long> res = new List<long>();
            res.Add(1);
            for (long i = 2; i <= n / 2 + 1; i++)
            {
                if (n % i == 0)
                {
                    res.Add(i);
                }
            }
            res.Add(n);
            return res.ToArray();
        }
        static bool isPrime(long num)
        {
            if (num == 1) { return false; }
            for (long i = 2; i <= Math.Sqrt(num) + 1; i++)
            {
                if (num % i == 0) { return false; }
            }
            return true;
        }

        static void CoutAr<Type>(IEnumerable<Type> types)
        {
            string text = string.Join(" ", types);
            WriteLine(text);
        }

        public static long nCr(int n, int r)
        {
            // naive: return Factorial(n) / (Factorial(r) * Factorial(n - r));
            return nPr(n, r) / Factorial(r);
        }

        public static long nPr(int n, int r)
        {
            // naive: return Factorial(n) / Factorial(n - r);
            return FactorialDivision(n, n - r);
        }

        private static long FactorialDivision(int topFactorial, int divisorFactorial)
        {
            long result = 1;
            for (int i = topFactorial; i > divisorFactorial; i--)
                result *= i;
            return result;
        }

        private static long Factorial(int i)
        {
            if (i <= 1)
                return 1;
            return i * Factorial(i - 1);
        }
    }

    public struct Pair<T>
    {
        public T a;
        public T b;
        public T c;
        public Pair(T _a, T _b) { a = _a; b = _b; c = default(T); }
        public Pair(T _a, T _b, T _c) { a = _a; b = _b; c = _c; }
    }
}