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 Julia (0.5.0) (judge verdict: Wrong Answer)

N=parse(Int,readline())
F=0
cnt=zeros(Int,26)
L=1
S=chomp(readline())
ans=0
for R=1:N
	F+=(cnt[Int(S[R])-96]+=1)>1
	while F>0
		F-=(cnt[Int(S[L])-96]-=1)==1
		L+=1
	end
	ans+=R-L+1
end
while L<N
	ans+=N-L+1
	L+=1
end
println(ans)