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 Ruby (2.3.3) (judge verdict: Accepted)

def cin; gets.split.map(&:to_i) end
def cout(*x); puts x.join(" ") end

exit if $0 != __FILE__

D = 10**9+7
n = gets.to_i
str = gets.chop
hs = {}
str.chars do |c|
  hs[c] = 0 if !hs[c]
  hs[c] += 1
end

r = hs.values.inject(1) {|cnt, x| (cnt * (x+1)) % D}

puts r-1