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;

fn main() {
    let mut buf = String::new();
    io::stdin().read_line(&mut buf).unwrap();
    let n = buf.trim().parse::<usize>().unwrap();
    let mut buf = String::new();
    io::stdin().read_line(&mut buf).unwrap();
    let s = buf.as_str();
    let mut pos = 0;
    let mut max = 0;
    for i in 0..n {
        let pre: HashSet<_> = s[0..i].chars().collect();
        let post: HashSet<_> = s[i..].chars().collect();
        let ovr = pre.intersection(&post).count();
        if max < ovr {
            pos = i;
            max = ovr;
        }
    }
    println!("{}", max);
}