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)

#![allow(non_snake_case)]

use std::io;
use std::collections::HashMap;
use std::collections::HashSet;
use std::cmp::max;

fn main() {
    let mut line = String::new();
    io::stdin().read_line(&mut line).unwrap();
    line.clear();
    io::stdin().read_line(&mut line).unwrap();

    let mut left = HashMap::new();
    let mut right = HashMap::new();
    let mut set = HashSet::new();
    let mut ret = 0;

    for c in line.chars() {
        *right.entry(c).or_insert(0) += 1;
    }

    for c in line.chars() {
        let remove_flag = {
            let x = right.get_mut(&c).unwrap();
            *x -= 1;
            *x == 0
        };
        if remove_flag {
            right.remove(&c);
            set.remove(&c);
        } else {
            set.insert(c);
        }
        *left.entry(c).or_insert(0) += 1;
        ret = max(ret, set.len());
    }

    println!("{}", ret);
}