code: public SparseGraph reversed() {
          SparseGraph reverse = new SparseGraph();
          for (Entry<Integer, Set<Integer>> entryFrom : edges.entrySet())
              for (Integer to : entryFrom.getValue())
                  reverse.addEdge(to, entryFrom.getKey());
          return reverse;
      }
docs: Build and return a sparse graph by reversing all edges in this graph.
  
  @return