binary tree, where each node has at most two child nodes, child nodes may contain references to their parents.
we do not differentiate the nodes and all nodes are considered identical. How can we find the number of different binary trees that can be formed with N identical nodes.
eg: if 3 nodes then 5 diff trees
if 7 nodes then 429 trees
Now, if you really want to understand this, instead of just getting (or experimenting to find) the answer, you can check out “The Art of Computer Programming”, Volume 4, Fascicle 4: Generating all trees.