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: Runtime Error)

n = int(input())

M = pow(10, 9) + 7
import sys

dc = {}
al = [0]

rr = [1]


def search(ss, start, end):
    z = al[(start + end)//2]
    nz = al[(start + end)//2+1]
    if z <= ss and ss < nz:
        return (start+end)//2
    elif ss < z:
        return search(ss, start, (start+end)//2)
    else:
        return search(ss, (start+end)//2+1, end)

c = 0
cn = 0
for line in sys.stdin:
    a = int(line)
    if a in dc:
        ss = dc[a]
        if ss != c-1:
            if ss >= al[cn]:
                rr.append((rr[cn]*2)%M)
            else:
                i = search(ss, 0, cn)
                rr.append((rr[cn] + rr[i])%M)
            al.append(c)
            cn += 1
    dc[a] = c
    c += 1
print(rr[cn]%M)