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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include "iostream"
#include "vector"
#include "string"
#include "algorithm"
#include "math.h"
#include "cstring"
#include "stack"
#include "queue"

using namespace std;
typedef long long ll;
typedef pair<int, int> pii;

ll mod = 1e9 + 7;

int main()
{
  int n;
  cin >> n;
  string s;
  cin >> s;
  ll a[26];
  for (int i = 0; i < 26; i++)
  {
    a[i] = 0;
  }
  for (int i = 0; i < s.size(); i++)
  {
    a[s[i] - 'a']++;
  }
  ll ans = 1;
  for (int i = 0; i < 26; i++)
  {
    ans *= a[i] + 1;
    ans %= mod;
  }
  cout << ans - 1 << "\n";
}