minimum spanning tree (connect all vertices in undirected graphic)

both are greedy algorithm

- kruskal algorithm, O(ElgV)

• make set for each vertex. set only contains one vertex
• sort G.E in non-descending order by weight of edge.
• for each edge(u,v), if they are not in same set, add edge(u,v) to A. and union u and v to one set. 

- prime algorithm O(ElgV) , 

• initialise all u.key=infinity max, u.pi = nil
• set root.key = 0
• make all nodes as a priority queue, root is in the first
• take node from the queue and update its key of adjacent nodes to w(u,v)
• repeat last step