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: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
class Scanner
{
    string[] s;
    int i;

    char[] cs = new char[] { ' ' };

    public Scanner() {
        s = new string[0];
        i = 0;
    }

    public string next() {
        if (i < s.Length) return s[i++];
        string st = Console.ReadLine();
        while (st == "") st = Console.ReadLine();
        s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
        if (s.Length == 0) return next();
        i = 0;
        return s[i++];
    }

    public int Int() {
        return int.Parse(next());
    }
    public int[] ArrayInt(int N, int add = 0) {
        int[] Array = new int[N];
        for (int i = 0; i < N; i++) {
            Array[i] = Int() + add;
        }
        return Array;
    }

    public long Long() {
        return long.Parse(next());
    }

    public long[] ArrayLong(int N, long add = 0) {
        long[] Array = new long[N];
        for (int i = 0; i < N; i++) {
            Array[i] = Long() + add;
        }
        return Array;
    }
}
class Program {
   
    static Scanner cin = new Scanner();
    static void Main() {
        int N = cin.Int();
        string S = Console.ReadLine();

        //char tmp = S[0];
        //for (int i = 1; i < N; i++) {
        //    if(S[i] == tmp)
        //}

        var alpa = new long[26];
        foreach (var x in S) {
            alpa[(int)(x - 'a')]++;
        }

        int cnt = 0;
        foreach (var x in alpa) {
            if (x > 0) cnt++;
        }
        if (cnt == 1) Console.WriteLine(1);
        else {
            long ans = 1;
            foreach (var x in alpa) {
                if (x > 0) ans = (ans % 1000000007 * (x + 1)) % 1000000007;
            }

            Console.WriteLine(ans > 0 ? ans - 1 : 1000000006);
        }

        //foreach (var x in alpa) {
        //    Console.WriteLine(x);
        //}
        Console.ReadLine();
    }
}