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: Runtime Error)

#!/usr/bin/env python
# -*- coding: utf-8 -*-


def main():
  n = int(input())
  prev = -1
  nums = []
  table = {}

  for _ in range(n):
    v = int(input())

    if v != prev:
      nums.append(v)
      prev = v

  for i, n in enumerate(nums):
    if n not in table:
      table[n] = []

    table[n].append(i)

  values = [0] * len(nums)
  value = 1

  for i in range(len(nums)):
    values[i] += value

    for j in table[nums[i]]:
      if j <= i:
        continue

      values[j] += value

    value = values[i]

  print(value % int(1e+9 + 7), flush=True)


if __name__ == '__main__':
  main()