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 Python (3.8.2) (judge verdict: Accepted)

n = int(input())
s = input()

alphabets = [chr(i + 97) for i in range(26)]

# forward
l1 = [0] * (n-1)
now = [False] * 26
for i in range(n-1):
    now[ord(s[i]) - 97] = True
    l1[i] = now.copy()

# backward
l2 = [0] * (n-1)
now = [False] * 26
for i in range(n-1, 0, -1):
    now[ord(s[i]) - 97] = True
    l2[i-1] = now.copy()

ans = 0
for i in range(n-1):
    cnt = 0
    for j in range(26):
        if l1[i][j] and l2[i][j]:
            cnt += 1

    if ans < cnt:
        ans = cnt

print(ans)