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 Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.util.*;

class Main{
    static Scanner sc = new Scanner(System.in);
    public static void main(String args[]){
        int N=sc.nextInt(),max=0;
        String S=sc.next();
        HashMap<String,Integer>x=new HashMap<>(),y=new HashMap<>();
        for(int i=0;i<N;i++){
            String T=String.valueOf(S.charAt(i));
            if(!x.containsKey(T)){
                x.put(T,1);
            }else{
                x.put(T,x.get(T)+1);
            }
        }
        for(int i=0;i<N;i++){
            String T=String.valueOf(S.charAt(i));
            if(!y.containsKey(T)){
                y.put(T,1);
            }
            else{
                y.put(T,y.get(T));
            }
            x.put(T,x.get(T)-1);
            if(x.get(T)==0){
                x.remove(T);
                y.remove(T);
            }
            max=Math.max(max,Math.min(x.size(),y.size()));
        }
        System.out.println(max);
    }
}