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;

namespace AtCoderBeginnerContest098
{
    class QB
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            string S = Console.ReadLine();
            HashSet<char> list = new HashSet<char>();
            int count = 0;
            int max = 0;
            for(int i = 0; i < N; i++)
            {
                //if (!list.Contains(S[i]))
                //{
                    if (S.IndexOf(S[i], i + 1) >= 0) count++;
                //}
                //else
                if (list.Contains(S[i]))
                {
                    if (S.LastIndexOf(S[i], i - 1) >= 0) count--;
                }
                list.Add(S[i]);
                if (max < count) max = count;
            }
            Console.WriteLine(max);
        }
    }
}