wish_me's blog

BFS with multiple sources
By wish_me, history, 7 years ago, In English

Can any one explain the topic and also tell me the list of some good problems.Thanks in advance.

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4 years ago, # |
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https://codeforces.net/contest/1272/problem/E

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4 years ago, # |
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Another way to think about multiple sources that's potentially nicer to code is to add a fake source with edges to all the real sources, then BFS as normal, then subtract 1 from all the distances.

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    4 years ago, # ^ |
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    can you please elaborate what are you trying to tell ?

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      4 years ago, # ^ |
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      Imagine this is your original graph (not including the red node/edges), and A, B, C are your multiple sources. We can create an additional node X and the red edges shown, then do a standard BFS starting from X. Finally, all the distances will be 1 more than they should be, because you had the extra hop from X to the real source at the beginning of each path.

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      4 years ago, # ^ |
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      • Instead of pushing all the real sources into the queue, you add a fake source, link it to all the real sources, and just push that single fake source into the queue.
      • If you do the BFS, it will take the fake source out of the queue first, and then it will push all the real sources into the queue right after (because you link the real sources directly to the fake source, they will be the closest to the fake source, which means they should be in the queue after the fake source is visited).
      • Now, all the real sources are inside the queue, as expected. The side effect is that the calculated distances are all +1 compared to the actual distances, that's because the fake source and the fake edges increase the distance. So, subtract 1 from all the distances, and you are good to go.
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        4 years ago, # ^ |
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        Sir, how does it affects the time complexity? does it make any difference at all.

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          4 years ago, # ^ |
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          I think there should be no huge differences. As our red coder said above, it's just nicer to code, as traditionally, you start with a single vertex only.

          If I misunderstand anything, please tell me.

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4 years ago, # |
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Another Problem: 986A - Fair

Add all the nodes you want to BFS from into the initial queue and run it.

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3 years ago, # |
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I love this community. Thanks for various approaches on multi source BFS.

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2 years ago, # |
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This is a nice problem illustrating the use of multiple source BFS: Monsters

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17 months ago, # |
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You should do "monsters" problem on cses.

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