Problem:
Score : 200 points

Problem Statement

You are given a string S of length N.
Among its subsequences, count the ones such that all characters are different, modulo 10^9+7. Two subsequences are considered different if their characters come from different positions in the string, even if they are the same as strings.

Here, a subsequence of a string is a concatenation of one or more characters from the string without changing the order.

Constraints

1 \leq N \leq 100000

S consists of lowercase English letters.

|S|=N

Input

Input is given from Standard Input in the following format:

N
S

Output

Print the number of the subsequences such that all characters are different, modulo 10^9+7.

Sample Input 1

4
abcd

Sample Output 1

15

Since all characters in S itself are different, all its subsequences satisfy the condition.

Sample Input 2

3
baa

Sample Output 2

5

The answer is five: b, two occurrences of a, two occurrences of ba. Note that we do not count baa, since it contains two as.

Sample Input 3

5
abcab

Sample Output 3

17

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)


//AGC031 A

import java.util.Arrays;
import java.util.Scanner;

class Main {
    static final int mod = 1000000007;

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        String s = sc.next();
        // dp[i] : i 番目まで
        int dp[] = new int[n + 1];
        boolean check[] = new boolean[26];
        int che[] = new int[30];
        Arrays.fill(check, false);
        for (int i = 1; i < n + 1; i++) {
            int a = (int) (s.charAt(i - 1)) - 97;
            if (che[a] == 0) {
                che[a]++;
                dp[i] = dp[i - 1] * 2 % mod + 1;

            } else {

                dp[i] = (dp[i - 1] - che[a]) / (che[a] + 1) * (che[a] + 2) % mod + che[a] + 1;
                che[a]++;
            }
        }
        System.out.println(dp[n]);

        sc.close();
    }
}