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

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.

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