title: Maximal path problem
body: <p>Given oriented unweighted graph and the problem is to find simple path of maximal length
  (start vertex and end vertex are not fixed). It obviously can be solved in O(n^2 * 2 ^n) but I heard that there is O(n * 2 ^ n) algorithm which I don't know. So how to solve it in O(n * 2 ^n)?    //n = |V|</p>