Figure 1
From: Efficient parallel and out of core algorithms for constructing large bi-directed de Bruijn graphs
This figure illustrates bi-directed between two nodes in a bi-directed graph. (a) Shows a bi-directed graph with five nodes {A, B, C, D, E}. (b) The alternating green, red edges show a path between node A and E. (c) Shows two valid bi-directed walks from node A to E, the first path is {A, B, E} and the second path is {A, B, C, D, B, E}, this also shows that the bi-directed path may not be simple and can contain repeating nodes - in this case node B.