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 Haskell (GHC 7.10.3) (judge verdict: Accepted)

import qualified Data.Map as M

main = do
  li <- getLine
  let n = read li
  s <- getLine
  let ans = compute n s
  print ans

{-
より数学チックに書きたいから書く
左はカウントする意味すらない。Setでいい。
-}

compute :: Int -> String -> Int
compute n s = maximum $ map comCnt $ scanl step (l0,r0) s
  where
    l0 = M.empty
    r0 = M.fromListWith (+) [ (c,1) | c <- s ]
    step (lm,rm) c = (M.insert c () lm, M.update decl c rm)
    decl 1 = Nothing
    decl n = Just (pred n)
    comCnt = M.size . uncurry (M.intersectionWith const)