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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;
int main(){
    int N;
    cin >> N;
    vector<char> S(N);
    for (int i = 0; i < N; i++){
        cin >> S.at(i);
    }
    set<char> st_x, st_y;
    int count = 0, count_f = 0;
    for (int i = 0; i < N; i++){
        for (int j = 0; j < i; j++){
            st_x.insert(S.at(j));
        }
        for (int j = i; j < N; j++){
            st_y.insert(S.at(j));
        }
        while (!st_x.empty()){
            char k = *begin(st_x);
            if (st_y.count(k)) count++;
            st_x.erase(k);
            //cout << "i " << i << " count " << count << " k " << k << endl;
        }
        while (!st_y.empty()){
            st_y.erase(*begin(st_y));
        }
        count_f = max(count, count_f);
        count = 0;
    }
    cout << count_f << endl;
}