How can one generate all unlabeled trees with $\le n$ nodes?
Visualization of all unlabeled trees with $\le6$ nodes:
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Start by considering rooted trees parametrised by vertex count and depth. These have an obvious recursive construction: a rooted tree of one vertex has depth $0$; a rooted tree of $n > 1$ vertices and depth $d \ge 1$ has a root and its children; at least one child has depth $d-1$, no child has depth $\ge d$, and the total number of vertices of the children is $n-1$.
Now, by a theorem of Jordan every tree either has a centre or bicentre. A tree with bicentre is formed by taking two rooted trees of equal depth and adding an edge between their roots. A tree with centre is a rooted tree where at least two children of the root have depth $d-1$.
Here is some python code https://gist.github.com/hagberg/7979081
I would start with all the trees size N-1 appending a new node to each of the nodes The tricky part is to identify whether each new tree is truly new and not isomorphic to previously generated trees. I'm guessing you need a system to code this topologies to detect symmetries. I'd try to find the longest path Longest path in an undirected tree with only one traversal and find a deterministic way to represent the graph
This now gone project to verify the graceful Tree conjecture contains some c++ code that might help you https://web.archive.org/web/20120729224449/http://www.eleves.ens.fr/home/wfang/gtv/index_en.html