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 PyPy2 (5.6.0) (judge verdict: Accepted)

#!/usr/bin/env python2
"""
This file is part of https://github.com/cheran-senthil/PyRival
Copyright 2019 Cheran Senthilkumar <hello@cheran.io>

"""
from __future__ import division, print_function

import itertools
import os
import sys
from atexit import register
from cStringIO import StringIO
from collections import Counter

range = xrange

filter = itertools.ifilter
map = itertools.imap
zip = itertools.izip

sys.stdout = StringIO()
register(lambda: os.write(1, sys.stdout.getvalue()))
input = StringIO(os.read(0, os.fstat(0).st_size)).readline

MOD = 10**9 + 7
MODF = float(MOD)

MAGIC = 6755399441055744.0
SHRT = 65536.0

MODF_INV = 1.0 / MODF
SHRT_INV = 1.0 / SHRT

fround = lambda x: (x + MAGIC) - MAGIC
fmod = lambda a: a - MODF * fround(MODF_INV * a)
fmul = lambda a, b, c=0.0: fmod(fmod(a * SHRT) * fround(SHRT_INV * b) + a * (b - SHRT * fround(b * SHRT_INV)) + c)


def main():
    n = input()
    s = input().rstrip('\r\n')

    res = 1
    for key, value in Counter(s).items():
        res = fmul(res, (value + 1))

    print(int(res - 1) % MOD)


if __name__ == '__main__':
    main()