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Figure 5-5.  A tree, drawn as a maze and as a more conventional graph diagram, superimposed on the maze
The left-hand rule is designed to be executed by an individual actually walking a maze, using only local 
information. To get a firm grip on what is really going on, we could drop this perspective and formulate the same 
strategy recursively.
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 Once you’re familiar with recursive thinking, such formulations can make it easier to see that an 
algorithm is correct, and this is one of the easiest recursive algorithms out there. For a basic implementation (which 
assumes one of our standard graph representations for the tree), see Listing 5-3.