code: public Map<Integer,Integer> getDistributionOfArticlesByCategory(Wikipedia pWiki, CategoryGraph catGraph) throws WikiPageNotFoundException {
          if (degreeDistribution == null) {   // has not been initialized yet
              iterateCategoriesGetArticles(pWiki, catGraph);
          }
          return degreeDistribution;
      }
docs: Computes the distribution of the number of articles per category.
  If the return value has been already computed, it is returned, else it is computed at retrieval time.
  @param pWiki The wikipedia object.
  @param catGraph The category graph.
  @return A map containing the distribution mapping from a degree to the number of times this degree is found in the category graph.
  @throws WikiPageNotFoundException