nE = 2 * nE;
        }

        checkEdges(nE);
    }

    private void checkVertices() {
        checkVertices(DEFAULT_VERTICES);
    }

    private void checkVertices(int nVertices) {
        if (vertices == null) {
            vertices = new TreeSet<>();
        }
    }

    private void checkEdges() {
        int count = DEFAULT_EDGES;