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;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main(string[] args) {
        int n = int.Parse(Console.ReadLine());
        string s = Console.ReadLine();
        Console.WriteLine(solve(n,s));
    }
    static int solve(int n, string s) {
        int[] ans = new int[n];
        for(int i = 1; i < n; i ++) {
            HashSet<char> x = new HashSet<char>(s.Substring(0,i));
            HashSet<char> y = new HashSet<char>(s.Substring(i));
            foreach(char c in x) {
                if(y.Contains(c)) ans[i] ++;
            }
        }
        return ans.Max();
    }
}