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)

#A問題
import collections
N=int(input())
S=str(input())
Slist=list(S)
count=0
maxn=2**N
for i in range(maxn):
    I=bin(i)
    I=I.lstrip("0b")
    Ilen=len(I)
    if Ilen<N:
        Id=N-Ilen
        j=0
        for j in range(Id):
            I="0"+I
    
    k=0
    klist=[]
    flag=0
    for k in range(N):
        if I[k]=="0":
            pass
        else:
            if Slist[k] in klist:
                flag=1
                break
            else:
                klist.append(Slist[k])
    if flag == 0 and len(klist)>0:
        count+=1
    count=count%1000000007
print(count)