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<math.h>
#include<map>
#include<iostream>
#include <algorithm>
#include <string>
#include<vector>
#include<array>
#include<tuple>
#include<queue>
#include<stack>
#include<iomanip>
#include<bitset>
#include <list>
#include<numeric>
//AtCoderC++.cpp

using namespace std;

inline int ceiling(int n, int d) {
	return n % d == 0 ? n / d : n / d + 1;
}

int main() {
	int N; string S; cin >> N >> S;
	auto left = vector<int>(26, 0), right = vector<int>(26, 0);
	for (size_t i = 0; i < N; i++) ++right[S[i] - 'a'];
	auto answer = 0, count = 0; int idx;
	for (size_t i = 0; i < N - 1; i++) {
		idx = S[i] - 'a';
		if (left[idx] == 0 && right[idx] > 1)++count;
		if (left[idx] > 0 && right[idx] == 1)--count;
		++left[idx]; --right[idx];
		answer = max(answer, count);
	}
	cout << answer << endl;
}