TITLE: Choosing two-colorable subgraph in a triangulation
QUESTION [7 upvotes]: Consider a planar graph $G$ which is a triangulation. 
Is it possible to find a two-colorable subgraph $H$ of $G$ which has a common edge with every face of $G$?
It is known that it is not always possible to take $H$ to be a spanning tree. See this MO question. 
Note that we do not require $H$ to be connected. 
If it is true, is there an efficient algorithm to find such a subgraph?

REPLY [14 votes]: Yes, such a subgraph always exist.  Let $G$ be a planar triangulation.  By the $4$-colour theorem, $G$ has a $4$-colouring.  We let $H$ be the subgraph consisting of all edges with endpoints coloured $1$ and $2$, or with endpoints coloured $3$ and $4$.  Since every face of $G$ is a triangle, every face must contain a $12$ edge or a $34$ edge, as required.  Also, $H$ is clearly bipartite since $(X,Y)$ is a bipartition of $H$ where $X$ is the set of vertices coloured $1$ or $3$ and $Y$ is the set of vertices coloured $2$ or $4$.  
Regarding the algorithmic question, there is a quadratic algorithm to find such a subgraph.  This follows from this paper of Robertson, Sanders, Seymour, and Thomas, where they present a quadratic algorithm to $4$-colour planar graphs.