title: efficient way to store graph with 26 millions edges
body: <p>I'm relatively new to graph processing and 
  I have a graph with 26 million edges along with nodes and I want to run k-shortest path algorithm on it, currently, I'm using a hashmap to store the edges, but I get java heap space out of memory error (which is understandable).</p>
  
  <p>So my question is what is the most efficient way to store this graph so that I don't get memory spill error and I can efficiently run k-shortest path algorithm.
  Is there some in-memory database specifically for graphs?</p>