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 OCaml (4.02.3) (judge verdict: Accepted)

(* O(n^3) *)
let n = read_int ()
let s = read_line ()
let ans = ref 0
let _ =
  for i = 1 to n - 1 do
    let x = String.sub s 0 i in
    let y = String.sub s i @@ n - i in
    let flags = Array.make 128 false in
    let count = ref 0 in
    let f c =
      if not flags.(int_of_char c) then
        (flags.(int_of_char c) <- true;
        try (String.index y c |> ignore; incr count) with
        | _ -> ()) in
    String.iter f x;
    ans := max !ans !count
  done;
  Printf.printf "%d\n" !ans