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 Python (3.4.3) (judge verdict: Time Limit Exceeded)

mod = 1000000007
n = int(input())
s = input()
abets ="abcdefghijklmnopqrstuvwxyz"
counts = []
for c in abets:
  counts.append(s.count(c))
ans =-1
for ptn in range(2** len(abets)):
  bin_str = format(ptn,'b')
  subans = 1
  for i, b in enumerate(bin_str[::-1]):
    if b =='1':
      subans = (subans * counts[i]) % mod
  ans = (ans + subans) % mod
print(ans)