Прошел первый этап XV Открытого Кубка по программированию. Как решать задачи А, Е, К?
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Прошел первый этап XV Открытого Кубка по программированию. Как решать задачи А, Е, К?
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A Let's compute answer for vertex v. We can visit an associative vertex u like this:
Firstly go some steps using given edges, after that go using reversed edges.
So Dfs(vertex, isReversed) will work perfect here.
From isReversed = false we can go to isReversed = true, but not in other way.
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K, dynamic programming and
meet in the middledivide and conquerSorry that I am in english.
Can you explain in more details problem K?
K dp(i, j) — first i keys are assigned, and we have covered first j alphabets.
dp(i, j) = min dp(i-1, k) + cost(k+1, j)
Let's denote optK(i, j) — optimal k that gives for us minimum.
We can prove that, optK(i, j) <= optK(i, j + 1)
let's calculate recursively calc(i, l..r, optKl, optKr).
Firstly, we can calculate dp(i, mid), then recursively solve for
calc(i, l..mid, optKl, opt(i, mid)) and calc(i, mid+1..r, opt(i,mid), optKr)).
321E - Ciel и гондолы also can be solved using this trick
Also, you can use convex hull optimization. Lets dp(i,j) — first i keys are assigned, and we have covered first j alphabets.
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K can be done by dp with a convex hull trick.
I would call it "divide and conquer" rather than "meet in the middle".
Indeed, thanks.