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: Wrong Answer)

# -*- coding: utf-8 -*-

import itertools

n = int(input())
s = str(input())

alphabet_map = {}
for i in range(n):
    if s[i] in alphabet_map.keys():
        alphabet_map[s[i]] += 1
    else:
        alphabet_map[s[i]] = 1

sum = 0
for i in range(len(alphabet_map.keys())):
    for comb in itertools.combinations(list(alphabet_map.keys()), i + 1):
        tmp = 1
        for x in comb:
            tmp *= alphabet_map[x]
        sum += tmp
print(sum)