The following algorithm M is a polynomial-time algorithm.

M = “On input ⟨G, s, t⟩, where G is a directed graph with nodes s and t:

1. Place a mark on node s.

2. Repeat the following until no additional nodes are marked:

3. Scan all the edges of G. If an edge (a, b) is found going from

   a marked node a to an unmarked node b, mark node b.

4. If t is marked, accept. Otherwise, reject.”