/**

     * Inserts an undirected edge between vertex positions v and w.

     * (No edge cost is assigned.) Time complexity: O(1).

     * 

     * @param  v    vertex position.

     * @param  w    vertex position.

     * @throws IllegalArgumentException if v or w are out of range.

     */

    public void addEdge(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));

            numEdges++;

            adjacentVertices[v].add(W);

            adjacentVertices[w].add(v);

            edgeCosts.put(new Edge(v,w),NO_COST);
	}

    }

    

    /**

     * Inserts an undirected edge with edge cost c between v and w.

     * Time complexity: O(1).

     * 

     * @param  c edge cost, c >= 0.

     * @param  v vertex.

     * @param  w vertex.

     * @throws IllegalArgumentException if v or w are out of range.

     * @throws IllegalArgumentException if c < 0.

     */

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

	if (!adjacentVertices[v].contains(W));

            numEdges++;

            adjacentVertices[v].add(W);

            adjacentVertices[w].add(v);

            edgeCosts.put(new Edge(v,w),c);
	}

    }