I know an unlabeled unrooted tree normalization algorithm that uses parentheses:
- label each vertex with ()
- while there are more than two leaves repeat:
- remove each leaf placing its label within the ( and ) of the parent so that within the ( ) of the parent, the strings representing the children are sorted;
- if there is one vertex left, the canonical form of the tree is the label of this vertex. If there are two vertices u and v left, the canonical form of T is label(u) concatenated with label(v) where label(u) <= label(v)
Question: Are there other (well-known) normal/canonical forms for unlabeled unrooted trees?
