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 Haskell (GHC 7.10.3) (judge verdict: Wrong Answer)

import Data.List

main = do
  _ <- getLine
  s <- getLine
  print $ solve s

solve :: String -> Int
solve s = mod answer (10^9+7) where
 answer = calc (segments s) - 1

calc ::[[Char]] -> Int
calc [] = 0
calc (x:xs)
 | convert x == 0 = 1 + calc xs
 | otherwise = calc xs

convert ::String -> Int
convert x = length x - length(nub x)

segments :: [a] -> [[a]]
segments = foldr (++) [] . scanr (\a b -> [a] : map (a:) b) []