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"
        "os"
        "strconv"
        "strings"
)

////////////////////////////////////////
///        templates                 ///
////////////////////////////////////////

var sc = bufio.NewScanner(os.Stdin)

func nextLine() string {
        sc.Scan()
        return sc.Text()
}

var rdr = bufio.NewReaderSize(os.Stdin, 1000000)

func readBigLine() string {
        buf := make([]byte, 0, 1000000)
        for {
                l, p, e := rdr.ReadLine()
                if e != nil {
                        panic(e)
                }
                buf = append(buf, l...)
                if !p {
                        break
                }
        }
        return string(buf)
}

func strSprit(str string) []string {
        cols := strings.Split(str, " ")
        return cols
}

func parseInt(str string) int {
        n, _ := strconv.Atoi(str)
        return n
}

func intSprit(str string) []int {
        strs := strSprit(str)
        cols := make([]int, len(strs))
        for i, v := range strs {
                cols[i] = parseInt(v)
        }
        return cols
}

type SortSlice []int

func (s SortSlice) Len() int {
        return len(s)
}

func (s SortSlice) Swap(i, j int) {
        s[i], s[j] = s[j], s[i]
}

func (s SortSlice) Less(i, j int) bool {
        return s[i] < s[j]
}

func reverseString(str string) string {
        buf := []rune(str)
        for i, j := 0, len(buf)-1; i < j; i, j = i+1, j-1 {
                buf[i], buf[j] = buf[j], buf[i]
        }
        return string(buf)
}

////////////////////////////////////////
///        end templates             ///
////////////////////////////////////////

func main() {
        line := nextLine()
        N := parseInt(line)

        str := nextLine()

        max := -1
        for i := 1; i < N; i++ {
                left := map[string]bool{}

                leftstr := str[:i]
                rightstr := str[i:]

                for _, s := range leftstr {
                        if _, ok := left[string(s)]; !ok {
                                left[string(s)] = true
                        }
                }

                count := 0
                for _, s := range rightstr {
                        if v, _ := left[string(s)]; v {
                                left[string(s)] = false
                                count++
                        }
                }
                if count > max {
                        max = count
                }
        }

        fmt.Println(max)

}