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 Pascal (FPC 2.6.2) (judge verdict: Accepted)

var
N,i:Longint;
S:String;
cnt:Array[1..26]of int64;
ans:int64=1;
begin
readln(N);
readln(S);
for i:=1 to N do inc(cnt[ord(S[i])-96]);
for i:=1 to 26 do ans:=ans*(cnt[i]+1)mod 1000000007;
writeln(ans-1);
end.