Kruskal's algorithm is a general-purpose algorithm for the minimum spanning tree problem, based on the disjoint sets data structure. The existence of very simple algorithms to maintain disjoint sets in almost constant time gives rise to simple implementations of Kruskal's algorithm whose running times are close to linear, usually outperforming Prim's algorithm in sparse graphs.
Theory of the algorithm
Kruskal's may be characterized as a greedy algorithm, which builds the MST one edge at a time. As befits a MST algorithm, the greedy strategy is to continually add the remaining edge of lowest weight. Unlike Prim's, however, Kruskal's adds edges without regard to the connectivity of the partially built MST; that is, it does not necessarily add an edge emanating from a vertex that is in the partially built MST. Indeed, it may be said that Kruskal's starts with forests of one vertex each, and adds edges one by one, each one causing two trees in the forest to coalesce into one, until all vertices have been placed in the same connected component and the MST is complete. In doing so, one must be careful not to add an edge between two vertices that are already in the same component, for doing so would create a cycle.