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>  // ver1.9
#define int long long
#define ALL(v) (v).begin(),(v).end()
#define Vi vector<int>
#define pb push_back
#define mp make_pair
#define cYES cout<<"YES"<<endl
#define cNO cout<<"NO"<<endl
#define cYes cout<<"Yes"<<endl
#define cNo cout<<"No"<<endl
#define rep(i,a,b) for(int i=a;i<b;i++)
#define repeq(i,a,b) for(int i=a;i<=b;i++)
#define repr(i,a,b) for(int i=a;i>b;i--)
#define repreq(i,a,b) for(int i=a;i>=b;i--)
#define leng(n) (int)(log10(n)+1)
#define INF 9000000000000000000  //  9.0 * 10^18
#define MOD 1000000007  //  10^9 + 7
#define PAI 3.14159265358979
using namespace std;

int gcd(int a,int b){return b?gcd(b,a%b):a;}  //  最大公約数gcd
int lcm(int a,int b){return a/gcd(a,b)*b;}  //  最小公倍数lcm

signed main() {
  cout << fixed << setprecision(14);
  int n;
  cin>>n;
  Vi a(26);
  char c;
  rep(i,0,n){
    cin>>c;
    a[c-'a']++;
  }
  int ans=1;
  rep(i,0,26){
    ans *= a[i]+1;
    ans %= MOD;
  }
  ans--;
  cout << ans << endl;
  return 0;
}