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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.InputStream;
import java.io.PrintStream;
import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;

public class Main {
	InputStream in = System.in;
	PrintStream out = System.out;

	private int kyotu(String a, String b) {
		Set<Character> ac = new HashSet<>();
		for (char c : a.toCharArray()) {
			ac.add(c);
		}
		Set<Character> bc = new HashSet<>();
		for (char c : b.toCharArray()) {
			bc.add(c);
		}

		ac.retainAll(bc);

		return ac.size();

	}

	public void _main(String[] args) {
		Scanner sc = new Scanner(in);

		int N = sc.nextInt();
		String S = sc.next();
		sc.close();

		int ans = 0;
		for (int i = 2; i < N; i++) {
			String a = S.substring(0, i);
			String b = S.substring(i);

			ans = Math.max(ans, kyotu(a, b));
		}
		out.println(ans);

	}

	public static void main(String[] args) {
		new Main()._main(args);
	}
}