Problem:
Score : 200 points

Problem Statement

You are given a string S of length N consisting of lowercase English letters.
We will cut this string at one position into two strings X and Y.
Here, we would like to maximize the number of different letters contained in both X and Y.
Find the largest possible number of different letters contained in both X and Y when we cut the string at the optimal position.

Constraints

2 \leq N \leq 100

|S| = N

S consists of lowercase English letters.

Input

Input is given from Standard Input in the following format:

N
S

Output

Print the largest possible number of different letters contained in both X and Y.

Sample Input 1

6
aabbca

Sample Output 1

2

If we cut the string between the third and fourth letters into X = aab and Y = bca, the letters contained in both X and Y are a and b.
There will never be three or more different letters contained in both X and Y, so the answer is 2.

Sample Input 2

10
aaaaaaaaaa

Sample Output 2

1

However we divide S, only a will be contained in both X and Y.

Sample Input 3

45
tgxgdqkyjzhyputjjtllptdfxocrylqfqjynmfbfucbir

Sample Output 3

9

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

using System;
using System.Collections.Generic;
using System.Linq;

using static System.Console;
using static System.Math;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            string S = Console.ReadLine();

            int maxType = 0;

            List<char> s = new List<char>();
            List<char> c1 = new List<char>();
            List<char> c2 = new List<char>();

            List<char> intersect = new List<char>();

            for (int i = 0; i < N; i++)
            {
                s.Add(S[i]);
            }

            for (int i = 1; i < s.Count; i++)
            {
                if (intersect.Count > N - i) break;

                c1.Clear();
                c2.Clear();
                intersect.Clear();

                c1 = s.Take(i).ToList();
                c2 = s.Skip(i).ToList();

                intersect = c1.Distinct().Intersect(c2.Distinct()).ToList<char>();
                
                if (intersect.Count > maxType)
                {
                    maxType = intersect.Count;
                }

            }
            Console.WriteLine(maxType);
        }

        static string[] Input() => System.Console.ReadLine().Split();
    }
}