Shayan's blog

Codeforces Educational Round 171 — Video Tutorial
By Shayan, history, 10 days ago, In English

Note: The text editorials will be provided by the authors of the round. This video tutorial acts as an additional resource for those who prefer video over text, not as a substitute for the text editorial.

2026A — Perpendicular Segments

Video

2026B — Black Cells

Video

2026C — Action Figures

Video

2026D — Sums of Segments

Video

2026E — Best Subsequence

Video
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10 days ago, # |
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Are there any ways to solve problem E with dp? I tried to solve it with dp but get WA.

Let dp[i][j] represent the value of mask such that the number of 1-bits in mask is minimized when constructing a subsequence of length j with i as the last element of the sequence.

I think we can combine with some greedy tactics, such as save more than one mask for each dp[i][j] at a time. Sorry for my bad English.

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    9 days ago, # ^ |
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    For your solution to passed, you have to store EVERY mask which satisfied your dp[i][j]. And since a[i] has at most 60 bits, i don't see how you will pass using this dp.

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      9 days ago, # ^ |
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      Because we have to store EVERY mask which satisfied dp[i][j], so i am curios that there are any ways to solve with probability/greedy?

      The first thing comes to my mind is dp[i][j] will save more than one mask at a time, we will limit the numbers of candidate about 10-20 candidates.

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        9 days ago, # ^ |
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        You can solve E by modelling it as a max flow question!

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          9 days ago, # ^ |
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          Can you explain how to model this problem into a maximum flow question? I see a lot of submissions getting AC with a maximum flow solution, but I don't understand how they convert the problem into maximum flow.

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            9 days ago, # ^ |
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            Try applying the Project Selection Problem with projects are array elements and machines are the 2-powers of their 1-bits.

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              9 days ago, # ^ |
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              Thank you, new knowledge has been acquired.

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9 days ago, # |
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Can anyone drop some good problems related to flows for practice. E seemed very doable after the flow thing

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    2 hours ago, # ^ |
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    There are 4 problems on cses in the graphs section which can be solved by network flows,

    https://cses.fi/problemset/task/1694 https://cses.fi/problemset/task/1711 https://cses.fi/problemset/task/1695 https://cses.fi/problemset/task/1696

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