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 Lua (5.3.2) (judge verdict: Accepted)

local n = io.read("*n", "*l")
local s = io.read()
local t = {}
for i = 1, 26 do t[i] = 0 end
for i = 1, n do
  local p = s:sub(i, i):byte() - 96
  t[p] = t[p] + 1
end
local ret = 1
for i = 1, 26 do
  ret = (ret * (1 + t[i])) % 1000000007
end
print(ret - 1)