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)

#include <bits/stdc++.h>
using namespace std;

long long N, ans, M, cnt[26];
char S;

int main() {
  M = 1e9 + 7;
  cin >> N;

  for (int i = 0; i < N; i++) {
    cin >> S;
    cnt[S - 'a']++;
  }

  ans = 1;
  for (int i = 0; i < 26; i++) {
    ans *= (cnt[i] + 1);
    ans %= M;
  }

  cout << ans - 1 << '\n';

  return 0;
}