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 System.Text;
using System.Text.RegularExpressions;

class Program
{
    static void Main(string[] args)
    {
        int N = int.Parse(Console.ReadLine());
        var S = Console.ReadLine();
        int max = 0;
        for (int i = 1; i < N - 1; i++)
        {
            var A = new List<char>();
            var B = new List<char>();
            for (int j = 0; j < N; j++)
            {
                if (j > i)
                {
                    if (!A.Contains(S[j]))
                    {
                        A.Add(S[j]);
                    }
                }
                else
                {
                    if (!B.Contains(S[j]))
                    {
                        B.Add(S[j]);
                    }
                }
            }
            max = Math.Max(max, A.Intersect(B).Count());
        }
        Console.WriteLine(max);
    }
}