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: Runtime Error)

use std::collections::HashSet;

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

fn chars(s: &str) -> HashSet<char> {
    let mut chars = HashSet::new();
    for c in s.chars() {
        chars.insert(c);
    }
    chars
}

fn same_chars(right_chars: HashSet<char>, left_chars: HashSet<char>) -> Vec<char> {
    let chars: Vec<char> = right_chars.iter().filter(|c| {
        right_chars.get(c).is_some() && left_chars.get(c).is_some()
    }).map(|c| *c).collect();
    chars
}

fn main() {
    let n: i8     = read();
    let s: String = read();
    let char_types_counts: Vec<i16> = (1 .. n - 1).map(|separate_index| {
        let (right, left) = s.split_at(separate_index as usize);
        let same_chars    = same_chars( chars(right), chars(left) );
        same_chars.len() as i16
    }).collect();
    println!("{}", char_types_counts.iter().max().unwrap());
}