Counting pairs when a person can form pair with at most one | Number of ways in which participant can take part . ; Base condition ; A participant can choose to consider ( 1 ) Remains single . Number of people reduce to ( x - 1 ) ( 2 ) Pairs with one of the ( x - 1 ) others . For every pairing , number of people reduce to ( x - 2 ) . ; Driver code
import java . io . * ; class GFG { static int numberOfWays ( int x ) { if ( x == 0 x == 1 ) return 1 ; else return numberOfWays ( x - 1 ) + ( x - 1 ) * numberOfWays ( x - 2 ) ; } public static void main ( String [ ] args ) { int x = 3 ; System . out . println ( numberOfWays ( x ) ) ; } }