Pre traverse fix #38
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Pre traverse fix #38
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added 3 commits
June 28, 2014 20:15
There are special cases such as this:
:a
/ | \
v v v
:e :b :c
| ^
| /
| /
| /
| /
|/
v
:d
in which the function yields false traverse. The error comes from the
fact that after discovering :a the algorithm marks its direct successors
:e, :b and :c as discovered. Therefore if the algorithm starts at :a
then chooses to visit :e, then :d it will start backtracking instead of
visiting :c because the vertex :c is marked as visited. If the
successors of :a are visited in the order (:e :b :c) then the result
would be (:a :e :d :b :c), which is obviously incorrect.
|
Great catch! Thanks for the fix and the test case revealing it. |
ghost
deleted the
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Hi!
After reading the code of pre-traverse i thought that there was a problem and i think was able to find an example that proves it.
The problem i saw was that after visiting a node the algorithms was marking all of its successors as visited(it put them in the seen set), which is common for BFS but not DFS. Therefore if we take that example graph:
and start the traversal from :a, what the algorithm will do is actually:
So it produces the result (:a :e :d :b :c) which is incorrect.
I chose the names of the vertices specifically for this example in order to force the algorithm to go through the branches in this exact order, otherwise it could just go through the branch from :e, go to :d, backtrack to :a and then choose :c and then :b. In that case the result is (:a :e :d :c :b) which is correct but the :c vertex was not visited when it had to be.