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.Linq;
using System.Collections;
using System.Collections.Generic;
using static Library.Comfort;
using static Library.Math;
using static Library.Algorithm;
using static System.Math;

namespace AtCoderer
{
    public class App
    {
        public static void Solve()
        {
            int n = int.Parse(Console.ReadLine());
            var s = Console.ReadLine().Select(v => (v - '0')).ToArray();
            int[,] dp = new int[n + 1, 3];
            int?[] memory = new int?[10];
            for (int i = 0; i < n; i++)
            {
                dp[i + 1, 0] = dp[i, 0] + (memory[s[i]] == null ? 1 : 0);
                dp[i + 1, 1] = dp[i, 0] + dp[i, 1] - dp[memory[s[i]] ?? 0, 0];
                dp[i + 1, 2] = dp[i, 1] + dp[i, 2] - dp[memory[s[i]] ?? 0, 1];
                memory[s[i]] = i;
            }
            Console.WriteLine(dp[n, 2]);
        }

        public static void Main(string[] args)
        {
#if DEBUG
            while (true)
            {
                Solve();
                Console.WriteLine();
            }
#else
            Solve();
#endif
        }
    }
}
namespace Library
{
    public static class Comfort
    {
        public const int mod = 1000000007;
        public class Point
        {
            public int a;
            public int b;
            public Point(int a, int b)
            {
                this.a = a;
                this.b = b;
            }
        }
        public class Triplet
        {
            public int a;
            public int b;
            public double c;
            public Triplet(int a, int b, double c)
            {
                this.a = a;
                this.b = b;
                this.c = c;
            }
        }
        public static int[] OrderBy(this int[] IArr)
        {
            int[] arr = new int[IArr.Length];
            Array.Copy(IArr, arr, IArr.Length);
            Array.Sort(arr);
            return arr;
        }
        public static int Max(params int[] p) => p.Max();
        public static long Max(params long[] p) => p.Max();
        public static int Min(params int[] p) => p.Min();
        public static long Min(params long[] p) => p.Min();
        public static int In()
        {
            return int.Parse(Console.ReadLine());
        }
        public static int Inl()
        {
            return int.Parse(Console.ReadLine());
        }
        public static int[] Parsed;
        public static int[] Parse()
        {
            Parsed = Console.ReadLine().Split().Select(v => int.Parse(v)).ToArray();
            return Parsed;
        }
        public static long[] Parsel()
        {
            return Console.ReadLine().Split().Select(v => long.Parse(v)).ToArray();
        }
        public static void Parse(out int a, out int b)
        {
            var r = Console.ReadLine().Split().Select(v => int.Parse(v)).ToArray();
            a = r[0];
            b = r[1];
        }
        public static void Parse(out int a, out int b, out int c)
        {
            var r = Console.ReadLine().Split().Select(v => int.Parse(v)).ToArray();
            a = r[0];
            b = r[1];
            c = r[2];
        }
        public static void Parse(out int a, out int b, out int c, out int d)
        {
            var r = Console.ReadLine().Split().Select(v => int.Parse(v)).ToArray();
            a = r[0];
            b = r[1];
            c = r[2];
            d = r[3];
        }
        public static void Parse(out long a, out long b)
        {
            var r = Console.ReadLine().Split().Select(v => long.Parse(v)).ToArray();
            a = r[0];
            b = r[1];
        }
        public static void Parse(out long a, out long b, out long c)
        {
            var r = Console.ReadLine().Split().Select(v => long.Parse(v)).ToArray();
            a = r[0];
            b = r[1];
            c = r[2];
        }
        public static void Parse(out long a, out long b, out long c, out long d)
        {
            var r = Console.ReadLine().Split().Select(v => long.Parse(v)).ToArray();
            a = r[0];
            b = r[1];
            c = r[2];
            d = r[3];
        }
        public static string ToString(this IEnumerable<char> v)
        {
            return string.Join("", v);
        }
        public static string Join<T>(this IEnumerable<T> v)
        {
            return string.Join(" ", v);
        }
        public static string Join<T>(this IEnumerable<T> v, char c)
        {
            return string.Join(c.ToString(), v);
        }
        public static string Join<T>(this IEnumerable<T> v, string s)
        {
            return string.Join(s, v);
        }
        public static object cout
        {
            set
            {
                Console.WriteLine(value);
            }
        }
        public static object dbg
        {
            set
            {
#if DEBUG
                Console.WriteLine(value);
#endif
            }
        }
        public static object dout
        {
            set
            {
                dbg = value;
            }
        }
    }
    public static class Math
    {
        public static bool IsNatural(this float v) => v - (int)v == 0;
        public static bool IsNatural(this double v) => v - (int)v == 0;
        public static long nCm(long n, long m)
        {
            if (m == 0) return 1;
            if (n == 0) return 0;
            return n * nCm(n - 1, m - 1) / m;
        }
        public static IEnumerable<int> Factor(int n)
        {
            int i = 2;
            int tmp = n;

            while (i * i <= n)
            {
                if (tmp % i == 0)
                {
                    tmp /= i;
                    yield return i;
                }
                else
                {
                    i++;
                }
            }
            if (tmp != 1) yield return tmp;
        }
        public static int Gcd(int a, int b) => a > b ? GcdRecursive(a, b) : GcdRecursive(b, a);
        private static int GcdRecursive(int x, int y) => y == 0 ? x : GcdRecursive(y, x % y);
        public static long Gcd(long a, long b) => a > b ? GcdRecursive(a, b) : GcdRecursive(b, a);
        private static long GcdRecursive(long x, long y) => y == 0 ? x : GcdRecursive(y, x % y);
        public static long Lcm(int a, int b) => ((long)a) * b / Gcd(a, b);
    }
    public static class Algorithm
    {
        public static IEnumerable<KeyValuePair<T, int>> UniteNext<T>(this IEnumerable<T> arr) where T : IEquatable<T>
        {
            int cont = 0;
            T prev = arr.First();
            foreach (var v in arr)
            {
                if (v.Equals(prev))
                {
                    cont++;
                }
                else
                {
                    yield return new KeyValuePair<T, int>(prev, cont);
                    cont = 1;
                }
                prev = v;
            }
            yield return new KeyValuePair<T, int>(prev, cont);
        }
        public class EnumAll
        {
            bool Over = false;
            int UpperValue;
            int Dimension;
            public int[] Current { private set; get; }
            public EnumAll(int MaxValue, int Dimension)
            {
                this.Dimension = Dimension;
                UpperValue = MaxValue;
                Current = new int[Dimension];
                Current[0] = -1;
            }
            public bool Next()
            {
                if (Over)
                    return false;
                bool flag = false;
                for (int i = 0; i < Dimension; i++)
                {
                    if (++Current[i] >= UpperValue)
                        Current[i] = 0;
                    else
                    {
                        flag = true;
                        break;
                    }
                }
                if (!flag)
                {
                    Over = true;
                    return false;
                }
                return true;
            }
        }
        public class Graph
        {
            public int Count = -1;
            private HashSet<int>[] Joined;
            public static Graph GenerateComplete(int size)
            {
                throw new NotImplementedException();
            }
            public Graph(int size)
            {
                Joined = new HashSet<int>[size];
                for (int i = 0; i < Joined.Length; i++)
                    Joined[i] = new HashSet<int>();
                Count = size;
            }
            public int[] GetConnected(int index)
            {
                return Joined[index].ToArray();
            }
            public void AddEdge(int a, int b)
            {
                if (a == b)
                    dbg = "ループを追加することはできません A:" + a + " B:" + b;
                else
                {
                    if (!Joined[a].Add(b))
                        dbg = "多重辺を追加することはできません A:" + a + " B:" + b;
                    else
                        Joined[b].Add(a);
                }
            }
            public int[] ToTree(int Root)
            {
                var ar = new int[Count];
                for (int i = 0; i < ar.Length; i++)
                    ar[i] = -10000000;
                ar[Root] = -1;
                Queue<int> bfs = new Queue<int>();
                bfs.Enqueue(Root);
                while (bfs.Count != 0)
                {
                    var t = bfs.Dequeue();
                    foreach (var v in Joined[t])
                    {
                        bfs.Enqueue(v);
                        if (ar[v] == -10000000)
                            ar[v] = t;
                    }
                }
                return ar;
            }
        }
        public class DiGraph
        {
            public int Count = -1;
            private HashSet<int>[] Joined;
            public DiGraph(int size)
            {
                Joined = new HashSet<int>[size];
                for (int i = 0; i < Joined.Length; i++)
                    Joined[i] = new HashSet<int>();
                Count = size;
            }
            public int[] GetConnected(int index)
            {
                return Joined[index].ToArray();
            }
            public void AddEdge(int a, int b)
            {
                if (a == b)
                    dbg = "ループを追加することはできません A:" + a + " B:" + b;
                else
                    Joined[a].Add(b);
            }
        }
        public class UnionFind
        {
            private int[] Parent;
            private int GetRoot(int v)
            {
                if (Parent[v] < 0)
                    return v;
                else
                    return Parent[v] = GetRoot(Parent[v]);
            }
            public UnionFind(int size)
            {
                Parent = new int[size];
                for (int i = 0; i < size; i++)
                    Parent[i] = -1;
            }
            public bool AreGrouped(int a, int b)
            {
                return GetRoot(a) == GetRoot(b);
            }
            public void Connect(int a, int b)
            {
                a = GetRoot(a);
                b = GetRoot(b);
                if (a == b) return;
                if (Parent[b] < Parent[a])
                {
                    Parent[b] += Parent[a];
                    Parent[a] = b;
                }
                else
                {
                    Parent[a] += Parent[b];
                    Parent[b] = a;
                }
            }
            public int GetGroupSize(int a)
            {
                return -Parent[GetRoot(a)];
            }
        }
        public class PriorityQueue<T> where T : IComparable
        {
            List<T> buffer = new List<T>();
            static void PushHeap(List<T> array, T elem)
            {
                int n = array.Count;
                array.Add(elem);

                while (n != 0)
                {
                    int i = (n - 1) / 2;
                    if (array[n].CompareTo(array[i]) < 0)
                    {
                        T tmp = array[n]; array[n] = array[i]; array[i] = tmp;
                    }
                    n = i;
                }
            }
            static void PopHeap(List<T> array)
            {
                int n = array.Count - 1;
                array[0] = array[n];
                array.RemoveAt(array.Count - 1);

                for (int i = 0, j; (j = 2 * i + 1) < n;)
                {
                    if ((j != n - 1) && (array[j].CompareTo(array[j + 1]) > 0))
                        j++;
                    if (array[i].CompareTo(array[j]) > 0)
                    {
                        T tmp = array[j]; array[j] = array[i]; array[i] = tmp;
                    }
                    i = j;
                }
            }
            public int Count
            {
                get
                {
                    return buffer.Count;
                }
            }
            public void Push(T elem)
            {
                PushHeap(this.buffer, elem);
            }
            public T Peek()
            {
                return buffer[0];
            }

            public T Pop()
            {
                T a = buffer[0];
                PopHeap(this.buffer);
                return a;
            }
        }
        public class KMP
        {
            int[] Table;
            string a, b;
            int m = 0;
            int i = 0;
            public KMP(string a)
            {
                this.a = a;
                Table = new int[a.Length];
                int subindex = 0;

                Table[0] = -1;
                Table[1] = 0;

                for (int i = 2; i < a.Length;)
                {
                    if (a[i - 1] == a[subindex])
                    {
                        subindex++;
                        Table[i] = subindex;
                        i++;
                    }
                    else if (subindex > 0)
                        subindex = Table[subindex];
                    else
                    {
                        Table[i] = 0;
                        i++;
                    }
                }
            }
            public void SetSearchBase(string v)
            {
                b = v;
                m = 0;
                i = 0;
            }
            public int Search()
            {
                m++;
                i = 0;
                while (m + i < b.Length)
                {
                    if (a[i] == b[m + i])
                    {
                        i++;
                        if (i == a.Length)
                            return m;
                    }
                    else
                    {
                        m = m + i - Table[i];
                        if (i > 0)
                            i = Table[i];
                    }
                }
                return -1;
            }
        }
    }
}