# Difference between revisions of "TADM2E 5.2"

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The RPO toposort should be : A B D E C F H G I J | The RPO toposort should be : A B D E C F H G I J | ||

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+ | --[[User:Someguy|Someguy]] ([[User talk:Someguy|talk]]) 04:27, 4 March 2016 (EST) | ||

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+ | Hi Ohk, thanks for the reply. | ||

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+ | The original graph on page 185 in the book pdf (obtained from "sist.sysu.edu.cn/~isslxm/DSA/textbook/Skiena.-.TheAlgorithmDesignManual.pdf") contains a directed cycle (G->F->H->G). According to the errata ("www3.cs.stonybrook.edu/~skiena/algorist/book/errata"), we should "reverse the edge (F,H)" to make it acyclic; that is to say changing the edge from F->H to H->F. It is different from yours (from H->F to F->H). | ||

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+ | As I understand, after the change either J or F could be the terminal vertex, since both vertices have no outgoing edges. A and H have no incoming edges, so they are unreachable from other vertices. |

## Revision as of 09:27, 4 March 2016

Be sure to apply the errata and change the direction of the edge between F and H.

A topological sorting of this graph would be:

A, B, D, E, C, H, G, I, J, F

--Someguy (talk) 05:13, 3 March 2016 (EST)

After the direction change of the edge between F and H, vertex H has no incoming edges, just like vertex A. If we start DFS at vertex A, according to the "topsort" on page 181, vertex H is supposed to appear at the beginning of the sorting, rather than in the middle:

H, A, B, D, E, C, G, I, J, F

Please correct me if I am wrong.

--Ohk (talk) 08:03, 3 March 2016 (EST)

If the direction from H->F is changed to F->H, then J should be the terminal vertex ?

A pre-order DFS walk should give the ordering : A B C F H G I J D E

The RPO toposort should be : A B D E C F H G I J

--Someguy (talk) 04:27, 4 March 2016 (EST)

Hi Ohk, thanks for the reply.

The original graph on page 185 in the book pdf (obtained from "sist.sysu.edu.cn/~isslxm/DSA/textbook/Skiena.-.TheAlgorithmDesignManual.pdf") contains a directed cycle (G->F->H->G). According to the errata ("www3.cs.stonybrook.edu/~skiena/algorist/book/errata"), we should "reverse the edge (F,H)" to make it acyclic; that is to say changing the edge from F->H to H->F. It is different from yours (from H->F to F->H).

As I understand, after the change either J or F could be the terminal vertex, since both vertices have no outgoing edges. A and H have no incoming edges, so they are unreachable from other vertices.