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)

N=int(input())
S=list(map(lambda x: ord(x),input()))
MOD = 10e9 + 7


# 各文字列について、出現回数+1(1つ取るか、一切とらない)
# それを全部かければ良い

AppearCount = [ 0 for _ in range(26)]

for n in range(N):
	AppearCount[S[n]-ord('a')] += 1

ans = 1

for n in range(N):
	if AppearCount[S[n]-ord('a')] != 0:
		ans *= (AppearCount[S[n]-ord('a')]+1)
		if MOD <= ans:
			ans -= MOD

print(ans-1)