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;
class Program
{
	static void Main(string[] args)
	{
		string t = Console.ReadLine();
		int N = int.Parse(t);
		string S = Console.ReadLine();
		string A = "abcdefghijklmnopqrstuvwxyz";
		int [] B = new int[26];
		int [] C = new int[26];
		for(var i=0;i<S.Length;i++){
			B[A.IndexOf(S.Substring(i,1))] += 1;
		}
		int countmax = 0;
		for(var i=0;i<S.Length-1;i++){
			B[A.IndexOf(S.Substring(i,1))] -= 1;
			C[A.IndexOf(S.Substring(i,1))] += 1;
			int count = 0;
			for(var j=0;j<26;j++){
				if(B[j] > 0 && C[j] > 0){
					count += 1;
				}
			}
			countmax = Math.Max(countmax,count);
		}
		Console.WriteLine(countmax);
	}
}