General techniques for comparing unrooted evolutionary trees

This paper presents two sets of techniques for comparing unrooted evolutionary trees, namely, label compression and four-way dynamic programming. The technique of four-way dynamic programming transforms existing algorithms for computing rooted maximum agreement subtrees into new ones for unrooted trees. Let n be the size of the two input trees. This technique leads to an O(n log n)-time algorithm for unrooted trees whose degrees are bounded by a constant, matching the best known complexity for the rooted binary case. The technique of label compression is not based on dynamic programming. With this technique, we obtain an O(n^1.5 log n)-time algorithm for unrooted trees with arbitrary degrees, also matching the best algorithm for the rooted unbounded degree case.