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 Python (3.4.3) (judge verdict: Wrong Answer)

import sys
MAX_INT = int(10e10)
MIN_INT = -MAX_INT
def IL(): return list(map(int,input().split()))
def SL(): return input().split()
def I(): return int(sys.stdin.readline())
def S(): return input()

def cnt(l):
  num = 1
  for i in l:
    if i != s:
      num *= (i+1)
  return num

N = I()
s = S()

dic = {"a":0,"b":0,"c":0,"d":0,"e":0,"f":0,"g":0,"h":0,"i":0,"j":0,"k":0,\
  "l":0,"m":0,"n":0,"o":0,"p":0,"q":0,"r":0,"s":0,"t":0,"u":0,"v":0,"w":0,\
    "x":0,"y":0,"z":0,}

ans = 0
for i in range(N):
  ans += cnt(list(dic.values()))//(dic[s[i]]+1)
  dic[s[i]] += 1

print(ans)