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.Linq;
using System.Collections.Generic;

public class Test
{
	public static void Main()
	{
		int N = int.Parse(Console.ReadLine());
		string s = Console.ReadLine();
		int Max = 0;
		HashSet<string> H = new HashSet<string>();
		int count = 0;
		for(int i = 0;i < N;i++){
		    for(int j = 0;j < i;j++){
		        if(!H.Contains(s[j].ToString()) && (s.Skip(i).Take(N-i).ToArray()).Any(a => a == s[j])){
		            H.Add(s[j].ToString());
		            count++;
		        }
		    }
		    if(count > Max)Max = count;
		    count = 0;
		    H.Clear();
		}
		Console.WriteLine(Max);
	}
}