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.Int32;

class Program
{
    static void Main(string[] args)
    {
        label:
        ReadLine();
        string txt = ReadLine();
        var dic = new Dictionary<char, KeyValuePair<int, int>>();
        for (int i = 0; i < txt.Length; ++i)
        {
            char c = txt[i];
            int key = dic.ContainsKey(c) ? dic[c].Key : i;
            dic[c] = new KeyValuePair<int, int>(key, i);
        }
        var list = dic.Values.Where(p => p.Key < p.Value).ToList();
        int result = 0;
        foreach (var pair in list)
        {
            int left = pair.Key;
            var count = list.Count(p => p.Key <= left && left < p.Value);
            result = Math.Max(result, count);
        }
        WriteLine(result);
    }
}