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: Accepted)

def sub_count(S,N,M):
    S_dict,S_set = {},set()

    for i in range(N):
        if S[i] not in S_set:
            S_set.add(S[i])
            S_dict[S[i]] = 1
        else:
            S_dict[S[i]] += 1

    output = 1
    for s in S_set:
        output *= (S_dict[s]+1)
        output %= M
    output -= 1
    return output

def main():
    N = int(input())
    S = input()
    M = 10**9+7

    ans = sub_count(S,N,M)
    print(ans)

if __name__ == "__main__":
    main()