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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.NoSuchElementException;

public class Main {

	final static int INF = Integer.MAX_VALUE;
	final static int MOD = 100000007;

	public static void main(String[] args) {
		new Main().run();
	}

	private void run() {
		FastScanner sc = new FastScanner();
		int n = sc.nextInt();
		char[] s = sc.next().toCharArray();
		int[] num = new int[n];
		for (int i = 0; i < n; i++) {
			num[i] = s[i] - '0';
		}
		int tmp;
		int tn;
		int cnt;
		int ans = 0;
		number: for (int i = 0; i <= 999; i++) {
			tmp = i;
			cnt = 0;
			keta: for (int j = 2; j >= 0; j--) {
				tn = tmp / (int) Math.pow(10, j);
				tmp %= (int) Math.pow(10, j);
				for (int k = cnt; k < n; k++) {
					if (tn == num[k]) {
						cnt = k + 1;
						continue keta;
					}
					if (k >= n - 1 - j) {
				//		System.out.println(i+","+j);
						continue number;
					}
				}
			}
			ans++;
		}

		PrintWriter out = new PrintWriter(System.out);
		out.println(ans);
		out.flush();

	}

	private int lowerbound(int[] a, int n) {//n<a[i]となる最小のi
		int l = 0;
		int r = a.length - 1;
		int c;
		if (n >= a[r]) {
			return a.length;
		}
		while (l != r) {
			c = (l + r) / 2;
			if (a[c] <= n) {
				l = c + 1;
			} else {
				r = c;
			}
		}
		return l;
	}

	private int upperbound(int[] a, int n) {//a[i]<nとなる最大のi
		int l = 0;
		int r = a.length - 1;
		int c;
		if (n <= a[0]) {
			return -1;
		}
		while (l != r) {
			c = (l + r + 1) / 2;
			if (a[c] < n) {
				l = c;
			} else {
				r = c - 1;
			}
		}
		return l;
	}

	private int lis(int[] a) {
		int[] b = new int[a.length + 1];
		Arrays.fill(b, INF);
		for (int i = 0; i < a.length; i++) {
			b[upperbound(b, a[i]) + 1] = a[i];
		}
		return upperbound(b, INF - 1) + 1;
	}

	private class FastScanner {
		private final InputStream in = System.in;
		private final byte[] buffer = new byte[1024];
		private int ptr = 0;
		private int buflen = 0;

		private boolean hasNextByte() {
			if (ptr < buflen) {
				return true;
			} else {
				ptr = 0;
				try {
					buflen = in.read(buffer);
				} catch (IOException e) {
					e.printStackTrace();
				}
				if (buflen <= 0) {
					return false;
				}
			}
			return true;
		}

		private int readByte() {
			if (hasNextByte())
				return buffer[ptr++];
			else
				return -1;
		}

		private boolean isPrintableChar(int c) {
			return 33 <= c && c <= 126;
		}

		public boolean hasNext() {
			while (hasNextByte() && !isPrintableChar(buffer[ptr]))
				ptr++;
			return hasNextByte();
		}

		public String next() {
			if (!hasNext())
				throw new NoSuchElementException();
			StringBuilder sb = new StringBuilder();
			int b = readByte();
			while (isPrintableChar(b)) {
				sb.appendCodePoint(b);
				b = readByte();
			}
			return sb.toString();
		}

		public long nextLong() {
			if (!hasNext())
				throw new NoSuchElementException();
			long n = 0;
			boolean minus = false;
			int b = readByte();
			if (b == '-') {
				minus = true;
				b = readByte();
			}
			if (b < '0' || '9' < b) {
				throw new NumberFormatException();
			}
			while (true) {
				if ('0' <= b && b <= '9') {
					n *= 10;
					n += b - '0';
				} else if (b == -1 || !isPrintableChar(b)) {
					return minus ? -n : n;
				} else {
					throw new NumberFormatException();
				}
				b = readByte();
			}
		}

		public int nextInt() {
			long nl = nextLong();
			if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE)
				throw new NumberFormatException();
			return (int) nl;
		}

		public double nextDouble() {
			return Double.parseDouble(next());
		}
	}
}