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 <bits/stdc++.h>

#define ll long long
#define ull unsigned long long

using namespace std;

const ll MOD = 1000000007;

int main() {
	ll N;
	cin >> N;
	string S;
	cin >> S;

	map<char, ll> m;
	for (int ii = 0; ii < N; ++ii){
		m[S[ii]] += 1;
	}

	ll ans = 1;
	for (char c = 'a'; c <= 'z'; ++c){
		ans = (ans * (m[c] + 1)) % MOD;
	}

	cout << ans - 1 << endl;

	return 0;
}