What is the reverse postorder?

Question

This is the explanation in wikipedia: Data-flow analysis

This is a typical iteration order for forward data-flow problems. In reverse-postorder iteration, a node is visited before any of its successor nodes has been visited, except when the successor is reached by a back edge. (Note that this is not the same as preorder.)

Can someone explain this in greater detail?

It feels like you really want to be posting in the CS forum: http://cs.stackexchange.com/ — DaFi4, Mar 21 '16 at 12:51
Have you tried [following the link in the article](https://en.wikipedia.org/wiki/Depth-first_search#Vertex_orderings)? — tobias_k, Mar 21 '16 at 18:26
Did the linked explanation of [Reverse postorder](https://en.wikipedia.org/wiki/Depth-first_search#Vertex_orderings) shed any light on it for you? — Jim Mischel, Mar 21 '16 at 18:26
This article has a diagram for postorder that's easier to understand: https://en.wikipedia.org/wiki/Tree_traversal — m69's been on strike for years, Mar 21 '16 at 21:48
@montewhizdoh Sorry, I don't know this forum before. I will next time ∩_∩. — lililqth, Mar 22 '16 at 02:41
@JimMischel What's the difference between the Level-order and the Reverse-postorder ? — lililqth, Mar 22 '16 at 09:02

score 33 · Accepted Answer · edited Dec 18 '17 at 13:05

33

Reverse postordering as the name suggests produces the exact opposite of postorder traversal.

Example

For the above referred directed graph

postorder traversals are D B C A and D C B A

reverse postorder traversals are A C B D and A B C D

How to obtain reverse postorder traversal

One way is to run postorder traversal and push the nodes in a stack in postorder.

Then pop out the nodes to get the reverse postorder.

Application

Topological sorting using depth first search.

edited Dec 18 '17 at 13:05

Marcel

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answered Dec 18 '17 at 11:03

Biboswan

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What is the different between reverse post-order DFS and BFS?! – Nov 11 '20 at 09:24
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@YasamanGhassemi for trees, they are the same (and also to pre-order DFS). For graphs (well, DAGs), they can be different if there are nodes that are both siblings and parent-child. – TemplateRex Dec 15 '20 at 18:57

What is the reverse postorder?

1 Answers1