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)

open Printf
open Scanf
open BatPervasives

(*open Core*)


(*let map2 f a1 a2 = Array.of_list (List.map2 f (Array.to_list a1) (Array.to_list a2))*)

let count (s1, s2) =
  let count_array s =
    let toi x = int_of_char x - int_of_char 'a' in
    let a = BatArray.create (toi 'z' + 1) 0 in
    BatString.iter (fun x -> a.(toi x) <- 1) s;
    a in
  let a1 = count_array s1 in
  let a2 = count_array s2 in
  let b = BatArray.map2 (fun a b ->
      match (a, b) with
      | (1, 1) -> 1
      | _ -> 0
    ) a1 a2 in
  BatArray.fold_left (+) 0 b

let split_string s i length =
  let s1 = String.sub s 0 i in
  let s2 = String.sub s i (length - i) in
  (s1, s2)

let main =
  let (length, s) = scanf "%d\n%s" (fun a b -> (a, b)) in
  let rec iter i v =
    match i with
    | i when i = length -> v
    | i -> iter (i + 1) (max (count (split_string s i length)) v) in
  let init = count (split_string s 1 length) in
  printf "%d\n" (iter 2 init)