dp array containing the sum
var dpArray = Array ( n + 1 ) . fill ( 0 ) ; dpArray [ 0 ] = 1 ; dpArray [ 2 ] = 1 ; for ( var i = 4 ; i <= n ; i += 2 ) { for ( var j = 0 ; j < i - 1 ; j += 2 ) { dpArray [ i ] += ( dpArray [ j ] * dpArray [ i - 2 - j ] ) ; } }