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 Python (3.4.3) (judge verdict: Runtime Error)

import itertools
def main():
    N = int(input())
    S = input()

    flags = dict_gen()
    counts = dict_gen()
    for i in S:
        flags[i] = 1
        counts[i] += 1

    counter = flag_counter(flags)
    print(counter)


def flag_counter(flags):
    alphabet = "abcdefghijklmnopqrstuvwxyz"
    counter = 0
    for i in alphabet:
        counter += flags[i]
    return counter    

def dict_gen():
    alphabet = "abcdefghijklmnopqrstuvwxyz"
    flags = {}
    for i in alphabet:
        flags[i] = 0
    return flags

def check

if __name__ == "__main__":
    main()