Number of ways a convex polygon of n + 2 sides can split into triangles by connecting vertices | Returns value of Binomial Coefficient C ( n , k ) ; Since C ( n , k ) = C ( n , n - k ) ; Calculate value of [ n * ( n - 1 ) * -- - * ( n - k + 1 ) ] / [ k * ( k - 1 ) * -- - * 1 ] ; A Binomial coefficient based function to find nth catalan number in O ( n ) time ; Calculate value of 2 nCn ; return 2 nCn / ( n + 1 ) ; Driver code
def binomialCoeff ( n , k ) : NEW_LINE INDENT res = 1 ; NEW_LINE if ( k > n - k ) : NEW_LINE INDENT k = n - k ; NEW_LINE DEDENT for i in range ( k ) : NEW_LINE INDENT res *= ( n - i ) ; NEW_LINE res /= ( i + 1 ) ; NEW_LINE DEDENT return res ; NEW_LINE DEDENT def catalan ( n ) : NEW_LINE INDENT c = binomialCoeff ( 2 * n , n ) ; NEW_LINE return int ( c / ( n + 1 ) ) ; NEW_LINE DEDENT n = 3 ; NEW_LINE print ( catalan ( n ) ) ; NEW_LINE