在C++中，Motzkin数
The Motzkin number series starts with 1, 1, 4, 9, etc.., We can get the generalised nth term with the sequence. The Motzkin number sequence is as follows.
a0 = 1
a1 = 1
a2 = 4
a3 = 9
an = ((2 * n + 1)/ n + 2) * M(n-1) +((3 * n - 3)/ n + 2) * M(n - 2)
Algorithm
Initialise the number n.
Iterate till n.
Update the previous two numbers
Return the last number.
