Convert a tree graph to its Prüfer sequenceSource:
to_prufer() converts a tree graph into its Prüfer sequence.
The Prüfer sequence of the graph, represented as a numeric vector of vertex IDs in the sequence.
The Prüfer sequence of a tree graph with n labeled vertices is a sequence of n-2 numbers, constructed as follows. If the graph has more than two vertices, find a vertex with degree one, remove it from the tree and add the label of the vertex that it was connected to to the sequence. Repeat until there are only two vertices in the remaining graph.
g <- make_tree(13, 3) to_prufer(g) #>  2 2 2 1 3 3 3 1 4 4 4