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++ (GCC 9.2.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
#define pb push_back
#define pi pair<int, int>
#define l first
#define r second
#define all(x) x.begin(), x.end()
#define fori(a, b, step) for (int i = a; i < b; i += step)
#define forj(a, b, step) for (int j = a; j < b; j += step)
#define int long long

const int maxn = 5e5 + 1, mod = 1e9 + 7;

using namespace std;

signed main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    string s;
    int n, cnt[26];
    cin >> n >> s;
    for (int i = 0; i < 26; i++)
        cnt[i] = 1;
    for (int i = 0; i < n; i++)
        ++cnt[s[i] - 'a'];
    int ans = 1;
    for (int i = 0; i < 26; i++)
        ans = (ans * cnt[i]) % mod;
    cout << (ans - 1 + mod) % mod;
    return 0;
}