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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;

namespace AGC031
{
    class As
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            string S = Console.ReadLine();
            long[] c = new long[26];
            long mod = 1000000007;

            for (int i = 0; i < N; i++)
            {
                c[S[i] - 'a']++;
            }

            for (int i = 0; i < c.Length; i++)
            {
                if (c[i] != 0) c[i]++;
            }

            //Console.WriteLine(string.Join(",", c));
            long res = 1;
            for (int i = 0; i < c.Length; i++)
            {
                if (c[i] != 0) res = (res * c[i]) % mod;
            }
            Console.WriteLine(res - 1);
        }
    }
}