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Friends Pairing

1. You are given a number n, representing the number of friends.

2. Each friend can stay single or pair up with any of it's friends.

3. You are required to print the number of ways in which these friends can stay single or pair up.

E.g.

1 person can stay single or pair up in 1 way.

2 people can stay singles or pair up in 2 ways. 12 => 1-2, 12.

3 people (123) can stay singles or pair up in 4 ways. 123 => 1-2-3, 12-3, 13-2, 23-1.

Input Format

A number n

Output Format

A number representing the number of ways in which n friends can stay single or pair up.

Constraints

0 <= n <= 20

Sample Input

4

Sample Output

10


Solution:

import java.io.*;

import java.util.*;


public class Main {


    public static void main(String[] args) throws Exception {

        // write your code here

        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();

        

        int[] dp = new int[n+1];

        dp[1] = 1;

        dp[2] = 2;

        for(int i=3;i<=n;i++)

            dp[i] = dp[i-1] + (i-1)*dp[i-2];

        

        System.out.println(dp[n]);

    }


}

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