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 Go (1.6) (judge verdict: Accepted)

package main

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strings"
)

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	var N int
	scanner.Scan()
	fmt.Sscanf(scanner.Text(), "%d", &N)

	var str string
	scanner.Scan()
	fmt.Sscanf(scanner.Text(), "%s", &str)
	chars := strings.Split(str, "")

	ans := 0
	for i := 0; i < N; i++ {
		left := make(map[string]struct{})
		for j := 0; j < i; j++ {
			left[chars[j]] = struct{}{}
		}
		right := make(map[string]struct{})
		for j := i; j < N; j++ {
			right[chars[j]] = struct{}{}
		}
		cnt := 0
		for key, _ := range left {
			if _, e := right[key]; e {
				cnt++
			}
		}
		ans = max(ans, cnt)
	}
	fmt.Println(ans)
}

func max(ns ...int) int {
	ret := math.MinInt64
	for _, n := range ns {
		if n > ret {
			ret = n
		}
	}
	return ret
}