Problem:
Score: 400 points

Problem Statement

AtCoder Inc. has decided to lock the door of its office with a 3-digit PIN code.

The company has an N-digit lucky number, S. Takahashi, the president, will erase N-3 digits from S and concatenate the remaining 3 digits without changing the order to set the PIN code.

How many different PIN codes can he set this way?

Both the lucky number and the PIN code may begin with a 0.

Constraints

4 \leq N \leq 30000

S is a string of length N consisting of digits.

Input

Input is given from Standard Input in the following format:

N
S

Output

Print the number of different PIN codes Takahashi can set.

Sample Input 1

4
0224

Sample Output 1

3

Takahashi has the following options:

Erase the first digit of S and set 224.

Erase the second digit of S and set 024.

Erase the third digit of S and set 024.

Erase the fourth digit of S and set 022.

Thus, he can set three different PIN codes: 022, 024, and 224.

Sample Input 2

6
123123

Sample Output 2

17

Sample Input 3

19
3141592653589793238

Sample Output 3

329

Solution in Ruby (2.3.3) (judge verdict: Runtime Error)

n = gets.chomp.to_i
s = gets.chomp.split(//).map(&:to_i)

def find_index_recursive(arr, char, prev_index)
  index = arr.find_index(char)
  return unless index
  if index < prev_index
    arr[index] = -1
    find_index_recursive(arr, char, prev_index)
  else
    index
  end
end

def main(n, s)
  sum = 0
  (0..9).each do |i|
    (0..9).each do |j|
      (0..9).each do |k|
        arr = s.clone

        index1 = arr.find_index(i)
        next unless index1
        arr[index1] = -1

        index2 = find_index_recursive(arr, j, index1)
        if !index2 || index2 < index1
          next
        end
        arr[index2] = -1

        index3 = find_index_recursive(arr, k, index2)
        if !index3 || index3 < index2
          next
        end
        arr[index3] = -1

        sum += 1
      end
    end
  end
  sum
end

puts main(n, s)


# require 'minitest/autorun'

# class FooTest < Minitest::Test
#   def test_case1
#     assert_equal(3, main(4, '0224'.split(//).map(&:to_i)))
#   end

#   def test_case2
#     assert_equal(17, main(6, '123123'.split(//).map(&:to_i)))
#   end

#   def test_case3
#     assert_equal(329, main(6, '3141592653589793238'.split(//).map(&:to_i)))
#   end
# end