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)

N = int(input())
S = input()

used_list = []
num = 0
a = 10**9 +7 

def DFS(i,used_list,num):
    if i == N:
        num += 1
        num = num % a
        return num

    s = S[i]
    # S[i]を使わない場合
    num = DFS(i+1,used_list,num) % a

    # S[i]を使う場合
    if s not in used_list:
        used_list.append(s)
        num=DFS(i+1,used_list,num) % a

        used_list.pop(-1)
    return num
    
       
ans = DFS(0,used_list,num)
ans = ans-1

print(ans)