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 REP(i, n) for(int i = 0; i < (int)n; i++)
#define REPvec(itr, mp) for(auto itr = mp.begin();itr != mp.end(); itr ++)
#define all(x) x.begin(), x.end()
#define MOD 1000000007//1e9+7
using namespace std;
typedef long long ll;
typedef vector<bool>    vecbool;
typedef vector<int>     vecint;
typedef vector<ll>      vecll;
typedef vector<string>  vecstr;
typedef vector<vecint>  vec2int;
typedef vector<vecbool> vec2bool;
int dx[4] {-1,+1, 0, 0};
int dy[4] { 0, 0,-1,+1};
int main()
{
  int n; cin >> n;
  string s; cin >> s;
  map<char, ll> mp;
  REP(i,n){
    mp[s[i]]++;
  }
  for(char c = 'a'; c <= 'z'; c++){
    mp[c] ++;
  }
  ll ans = 1;
  for(char c = 'a'; c <= 'z'; c++){
    ans *= mp[c];
    ans %= MOD;
  }
  if(ans == 0){
    ans = MOD-1;
  }else{
    ans -= 1;
  }
  cout << ans << endl;
}