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: Time Limit Exceeded)

# -*- coding: utf-8 -*-
"""
Created on Sat Mar 16 21:00:45 2019

@author: iwata
"""
import itertools

N = input()
S = input()

Sset = list(set(S)) 
Sset1 = []

a = 0
b = 0

for i in range(len(Sset)):
    num = S.count(Sset[i])
    Sset1.append(num)

for j in range(len(Sset)):
    com =  list(itertools.combinations(Sset1, j+1))
    for k in range(len(com)):
        for l in range(j+1):
            if l == 0:
                a = a + com[k][l]
            else:
                a = a * com[k][l]
        b = b + a
        a = 0

print(b%(10**9+7))