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 peopl reduce to ( x - 2 ) . ; Driver code
< ? php function numberOfWays ( $ x ) { if ( $ x == 0 $ x == 1 ) return 1 ; else return numberOfWays ( $ x - 1 ) + ( $ x - 1 ) * numberOfWays ( $ x - 2 ) ; } $ x = 3 ; echo numberOfWays ( $ x ) ; ? >