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 Go (1.6) (judge verdict: Accepted)

package main

import "fmt"

func main() {
	var N int
	fmt.Scan(&N)
	var S string
	fmt.Scan(&S)
	ans := 0
	// Sは最大30000なので、nC3から判定することは出来ない
	// ラッキーナンバーの候補は000から999までの1000パターンしかないので、これらを全探索出来ないか？
	// つまり、Nから3個取り出して、000から999までを作り出すことはできるか考えればよい？
	for pin := 0; pin <= 999; pin++ {
		pins := fmt.Sprintf("%03d", pin)
		ok1, ok2, ok3 := false, false, false
		t := pins[0]
		for i := 0; i < len(S); i++ {
			if !ok1 && S[i] == t {
				ok1 = true
				t = pins[1]
			} else if ok1 && !ok2 && S[i] == t {
				ok2 = true
				t = pins[2]
			} else if ok1 && ok2 && S[i] == t {
				ok3 = true
			}
		}
		if ok1 && ok2 && ok3 {
			ans++
		}
	}
	fmt.Println(ans)
}