Kruskal's Algorithm for finding Minimum Spanning Tree

MST.java

import java.util.ArrayList;
import java.util.Collection;
import java.util.PriorityQueue;

public class MST {

  /**
	 * Run Kruskal's algorithm on the given graph and return the MST, return
	 * null if no MST exists for the graph
	 * 
	 * @param g
	 *            the graph, g will never be null
	 * @return the MST of the graph, null of no valid MST exists
	 */
	public static Collection<Edge> kruskals(Graph g) {
		DisjointSets<Vertex> ds = new DisjointSets<Vertex>(g.getVertices());
		PriorityQueue<Edge> pq = new PriorityQueue<Edge>();
		ArrayList<Edge> mst = new ArrayList<Edge>();  
		for (Edge e : g.getEdgeList()) {
            		pq.add(e);
       		 }
        while (!pq.isEmpty() && mst.size() < g.getVertices().size() - 1) {
            Edge e = pq.poll();
            Vertex v = e.getV();
            Vertex u = e.getU();
            if (!ds.sameSet(u, v)) {
                ds.merge(v, u);
                mst.add(e);  
            }	
            if(g.getVertices().size()-1 == mst.size()){
        		return mst;
            }
        }
		return null;
	}
}