Approximate Forest Completion and Learning-Augmented Algorithms for Metric Minimum Spanning Trees

Finding a minimum spanning tree (MST) for n points in an arbitrary metric space is a fundamental primitive for hierarchical clustering and many other ML tasks, but this takes Ω(n 2 ) time to even approximate. We introduce a framework for metric MSTs that first (1) finds a forest of trees using practical heuristics, and then (2) finds a small weight set of edges to connect disjoint components in the forest into a spanning tree. We prove that optimally solving step (2) still takes Ω(n 2 ) time, but we provide a subquadratic 2.62approximation algorithm. In the spirit of learningaugmented algorithms, we then show that if the heuristic forest found in step (1) overlaps with an optimal MST, we can approximate the original MST problem in subquadratic time, where the approximation factor depends on a measure of overlap. In practice, we find nearly optimal spanning trees for a wide range of metrics, while being orders of magnitude faster than exact algorithms.