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::Read;
use std::collections::HashSet;

fn main() {
    let mut buf = String::new();
    std::io::stdin().read_to_string(&mut buf).unwrap();
    let mut it = buf.split_whitespace();
    let n: usize = it.next().unwrap().parse().unwrap();
    let tmp_s: String = it.next().unwrap().parse().unwrap();
    let s: Vec<char> = tmp_s.chars().collect::<Vec<char>>();

    let mut ans = 0;
    for i in 0..n {
        let mut kind = 0;
        let mut exist1 = HashSet::new();
        let mut exist2 = exist1.clone();
        for j in 0..i {
            exist1.insert(s[j]);
        }
        for j in i..n {
            exist2.insert(s[j]);
        }
        for c in exist1.iter() {
            if exist2.contains(c) {
                kind += 1;
            }
        }
        ans = std::cmp::max(ans, kind);
    }
    println!("{}", ans);
}