DFS
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A vertex has three states in DFS: undiscovered, discovered, finished.

The template for DFS:

.. code:: none

  dfs(r, p, graph, x)

  where

  r is the reference of full solution, which you want to return finally.
  p is partial solution, which is element in r.
  graph is graph you want to traverse.
  x is used for termination condition of dfs.