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 <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <stdbool.h>
typedef long long ll;
#define rep(i,n) for(ll i=0; i<(ll)(n); i++)
#define max(l,r) ((l)>(r)?l:r)
#define min(l,r) ((l)<(r)?l:r)
const ll MOD = 1000000007;

int main(){
    //input
    int n;
    scanf("%d", &n);
    char s[101010];
    scanf("%s", s);

    //solve
    ll ans = 1;
    int cnt[28] = {0};
    rep(i ,n) cnt[s[i] - 'a']++;
    rep(i, 26) {
        ans *= cnt[i]+1;
        ans %= MOD;
    }
    ans = (ans - 1) % MOD;

    //output
    printf("%lld", ans);
}