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

class Main {

    private static Scanner sc = new Scanner(System.in);

    public static void main(String[] args) {
        int n = sc.nextInt();
        String s = sc.next();
        Set<String> setA = new HashSet<>(), setB = new HashSet<>();

        int max = 0;
        for (int i = 1; i < n; i++) {
            int count = 0;
            String ss = s.substring(0, i);
            String st = s.substring(i, s.length());
            for (int j = 0; j < ss.length(); j++) {
                setA.add(ss.substring(j, j + 1));
            }
            for (int j = 0; j < st.length(); j++) {
                setB.add(st.substring(j, j + 1));
            }
            List<String> list = new ArrayList<>(setA);
            for (String str : list) {
                if (setB.contains(str)) {
                    count++;
                }
            }
            max = Math.max(max, count);
            setA.clear();
            setB.clear();
        }
        System.out.println(max);
    }
}