public void removeEdge(int v, int w) throws IllegalArgumentException {
        if (v < 0 || v >= numVertices || w < 0 || w >= numVertices) {
            throw new IllegalArgumentException(
                "Out of range: v = " + v + ", w = " + w + ".");
        }

	    if (adjacentVertices[v].contains(w) || adjacentVertices[w].contains(v)) {
            adjacentVertices[v].remove(w);
            adjacentVertices[w].remove(v);
            edgeCosts.remove(new Edge(v,w));
            numEdges--;
	    }
    }