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::str::FromStr;
use std::collections::HashSet;

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

fn main() {
  let n: i32 = input();
  let s: String = input();
  let mut max = 0;
  for i in 1..s.len() {
    let mut j = 0;
    let mut set1 = HashSet::new();
    let mut set2 = HashSet::new();
    for c in s.chars() {
      if j < i {
        set1.insert(c);
      } else {
        set2.insert(c);
      }
     j += 1;
    }
    let sum = set1.intersection(&set2).count();
    if sum > max {
      max = sum;
    }
    //println!("{:?}", set1);
    //println!("{:?}", set2);
    //println!("{}", sum);
  }
  println!("{}", max);
}