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.util.HashSet;
import java.util.Scanner;
import java.util.Set;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		char[] s = sc.next().toCharArray();

		Set<String> ans = new HashSet<String>();
		Set<String> x = new HashSet<String>();
		Set<String> y = new HashSet<String>();

		int max = 0;

		for (int i = 1; i < n; i++) {
			for (int ind = 0; ind < n; ind++) {
				if (ind < i) {
					x.add(s[ind] + "");
				} else {
					y.add(s[ind] + "");
				}
			}

			for (String ss : x) {
				if (y.contains(ss))
					ans.add(ss);
			}

			if (max < ans.size())
				max = ans.size();

			ans = new HashSet<String>();
			x = new HashSet<String>();
			y = new HashSet<String>();
		}

		System.out.println(max);

	}
}