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: Wrong Answer)

n=gets.to_i
s=gets.chomp
array=s.split("")
class Array
  def beki
    ret = []
    0.upto(2**size - 1) { |n|
      a = []
      each { |i|
        a.push i if n & 1 == 1
        n >>= 1
      }
      ret.push a
    }
    ret
  end
end
brray = array.beki
crray = brray.select{ |x| x == x.uniq }.map
puts crray.count