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 Rust (1.15.1) (judge verdict: Accepted)

use std::io::*;
use std::collections::HashSet;
use std::cmp;

fn read<T: std::str::FromStr>() -> T {
    let stdin = stdin();
    let mut buf = String::new();
	stdin.lock().read_line(&mut buf);
	buf.trim().parse().ok().unwrap()
}

fn read_vec_char() -> Vec<char> {
    read::<String>().chars().collect()
}

fn main() {
    let n = read::<usize>();
    let s = read_vec_char();
    let mut ans = 0;
    let mut s1: HashSet<char> = HashSet::new();
    let mut s2: HashSet<char>;
    for i in 1..n {
        s1.insert(s[i-1]);
        s2 = s[i..n].iter().map(|&x| x).collect();
        let m = s1.intersection(&s2).collect::<HashSet<_>>().len();
        ans = cmp::max(ans,m);
    }
    println!("{}",ans);
}