getNeighbours(vertexId = null) {
    if (vertexId === null) {
      return this.neighbours.slice();
    }

    let arr = Array();

    for (let i = 0; i < this.neighbours.length; i++) {
      if (this.neighbours[i] !== vertexId) {
        arr.push(this.neighbours[i]);
      }
    }

    return arr;
  }
Returns an array of ids of neighbouring vertices.

@param {Number} [vertexId=null] If a value is supplied, the vertex with this id is excluded from the returned indices.
@returns {Number[]} An array containing the ids of neighbouring vertices.