问题
I need a function which determines whether a path exists between vertices.
Input:
- undirected graph as a list
- two vertices
For example:
(is_it_a_path? '(2 ((1 2) (3 4))) 1 4) ;; returns true
The function also needs to be tail recursive.
How do I do this?
回答1:
The (free, online) textbook How To Design Programs has several sections that may be helpful to you.
You say that the solution must be tail-recursive. If you mean that all calls to the search procedure must be in tail position, then you're going to have to keep track of visited-nodes and paths-to-nodes explicitly.
Next: I'm confused by your example; it looks like the input is... a list of length two containing a goal node and some representation of the graph? But... no, I'm still confused.
You need to explain how what the input means---for instance, how are graphs represented as inputs to your function?
来源：https://stackoverflow.com/questions/9402362/determine-whether-undirected-graph-has-path-between-two-vertices