Prove that 2-Colourability is in L from Undir-Reachability is in L

Question

Let Undir-Reachability be the following problem: given an undirected graph G and two specified vertices s and t in G, is there a path from s to t in G?

I need to prove that the 2-Colourability is in L, by knowing that Undir-Reachability belongs to the complexity class L.

I don't know how to start.

Answer 1

Hint: a graph is not bipartite if there is a walk of odd length from a vertex to itself.