Boruvka

Computes the minimum spanning tree of an undirected weighted graph using Borůvka's algorithm.

The algorithm (historically the oldest MST algorithm, 1926):

1. Start with each vertex as its own component.
2. For each component, find the minimum-weight edge crossing to another component.
3. Add all such edges to the MST (merging components).
4. Repeat until only one component remains.

Each round halves the number of components, giving O(E log V) overall complexity.

NOTE: this is a direct iterative algorithm and does not use the Visitor traversal engine. The per-component minimum-edge scan in each round is embarrassingly parallel and could be parallelised using parallel collections or futures (TODO).

NOTE: intended for undirected weighted graphs only.