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 Go (1.6) (judge verdict: Wrong Answer)

package main

import (
  "fmt"
)

func main() {

  var N int
  fmt.Scan(&N)
  var S string
  fmt.Scan(&S)
  d := make(map[byte]int)
  for i := 0; i < N; i++ {
    d[S[i]]++
  }

  ans := 1
  for _, v := range d {
    ans *= 2 + v-1

  }
  ans = (ans - 1) % 1000000007
  fmt.Println(ans)
}