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.*;

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

    int ans = 0;
    for (int i = 0; i < n - 1; i++) {
      // 位置iを含む左半分を切断。出現文字の個数をカウント。
      int[] seen = new int[26];
      for (int j = 0; j <=i; j++) {
        int k = s.charAt(j) - 'a';
        seen[k]++;
      }

      // 右半分に対して、左半分で出現した文字をカウント。
      // 同じ文字を重複して数えないようにする。
      int cnt = 0;
      for (int j = i + 1; j < n; j++) {
        int k = s.charAt(j) - 'a';
        if (seen[k] != 0) {
          cnt++;
          seen[k] = 0;
        }
      }
      if (ans < cnt) ans = cnt; // 最大値更新
    }

    System.out.print(ans);
  } 
}