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)

from sys import exit
L = int(input())
# a = [int(input()) for _ in range(N)]
S = str(input())
dic = {}
for s in S:
    if not s in dic:
        dic[s] = 1
    else:
        dic[s]+=1
kind = list(dic.keys())
kind_num = len(kind)
ans = 0
for bit in range(1,(1 << kind_num)):
    tmp = 1
    for i in range(0,kind_num):
        if(bit & (1 << i)):
            tmp*=dic[kind[i]]
    ans+=tmp
print(ans)
exit()