public void addEdge(Value v1, Value v2) {
        HashSet<Value> v1Edges = adjacencyList.get(v1);
        HashSet<Value> v2Edges = adjacencyList.get(v2);
        if(v1Edges == null) {
            v1Edges = new HashSet<Value>();
            adjacencyList.put(v1, v1Edges);
        }
        if(v2Edges == null) {
            v2Edges = new HashSet<Value>();
            adjacencyList.put(v2, v2Edges);
        }
        v1Edges.add(v2);
        v2Edges.add(v1);
    }