title: bi-directional maximum flow using ford-fulkerson
body: <p>I think this is like an undirected graph version of max flow problem.</p>
  
  <p>So for every edge a->b, b->a is also valid. its bi-directional. And they share the same capacity. 
  Which means if I have capacity 10 between two vertex a, b , and I have a flow from a to b which costs 5, then the remaining capacity from a to b will be 5 as well as the remaining capacity from b to a.</p>
  
  <p>My solution to this is to have one directed edge from b to a and another one from a to b.
  The question is, if I decrease residual from a->b in residual graph, do I still increase the residual for the backward edge b->a? </p>