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 LuaJIT (2.0.4) (judge verdict: Accepted)

local n = io.read("*n", "*l")
local s = io.read()
local t1, t2, t3 = {}, {}, {}
for i = 1, 10 do
  t1[i] = false
end
for i = 1, 100 do
  t2[i] = false
end
for i = 1, 1000 do
  t3[i] = false
end
for i = 1, n do
  local k = tonumber(s:sub(i, i))
  if k == 0 then k = 10 end
  for j = 1, 100 do
    if t2[j] then
      t3[(j - 1) * 10 + k] = true
    end
  end
  for j = 1, 10 do
    if t1[j] then
      t2[(j - 1) * 10 + k] = true
    end
  end
  t1[k] = true
end
local ret = 0
for i = 1, 1000 do
  if t3[i] then ret = ret + 1 end
end
print(ret)