code: public Map<Integer,Integer> getVertexMapping() {
          Map<Integer,Integer> vertexMapping = new HashMap<Integer,Integer>();
          for (int i = 0, j = 0; i < n1; ++i) {
              if (core1[i] != NULL_NODE) {
                  vertexMapping.put(i, core1[i]);
              }
          }
          return vertexMapping;
      }
docs: {@inheritDoc}