View problem - Nile (IOI24_nile)

Time limit Memory limit # of submissions # of accepted Ratio
2000 ms 2048 MiB 159 35 22.01%

You want to transport $N$ artifacts through the Nile. The artifacts are numbered from $0$ to $N-1$. The weight of artifact $i$ ($0 \leq i < N$) is $W[i]$.

To transport the artifacts, you use specialized boats. Each boat can carry at most two artifacts.

  • If you decide to put a single artifact in a boat, the artifact weight can be arbitrary.
  • If you want to put two artifacts in the same boat, you have to make sure the boat is balanced evenly. Specifically, you can send artifacts $p$ and $q$ ($0 \leq p < q < N$) in the same boat only if the absolute difference between their weights is at most $D$, that is $|W[p] - W[q]| \leq D$.

To transport an artifact, you have to pay a cost that depends on the number of artifacts carried in the same boat. The cost of transporting artifact $i$ ($0 \leq i < N$) is:

  • $A[i]$, if you put the artifact in its own boat, or
  • $B[i]$, if you put it in a boat together with some other artifact.

Note that in the latter case, you have to pay for both artifacts in the boat. Specifically, if you decide to send artifacts $p$ and $q$ ($0 \leq p < q < N$) in the same boat, you need to pay $B[p] + B[q]$.

Sending an artifact in a boat by itself is always more expensive than sending it with some other artifact sharing the boat with it, so $B[i] < A[i]$ for all $i$ such that $0 \leq i < N$.

Unfortunately, the river is very unpredictable and the value of $D$ changes often. Your task is to answer $Q$ questions numbered from $0$ to $Q-1$. The questions are described by an array $E$ of length $Q$. The answer to question $j$ ($0 \leq j < Q$) is the minimum total cost of transporting all $N$ artifacts, when the value of $D$ is equal to $E[j]$.

Implementation Details

You should implement the following procedure.

std::vector<long long> calculate_costs(
    std::vector<int> W, std::vector<int> A, 
    std::vector<int> B, std::vector<int> E)
  • $W$, $A$, $B$: arrays of integers of length $N$, describing the weights of the artifacts and the costs of transporting them.
  • $E$: an array of integers of length $Q$ describing the value of $D$ for each question.
  • This procedure should return an array $R$ of $Q$ integers containing the minimum total cost of transporting the artifacts, where $R[j]$ gives the cost when the value of $D$ is $E[j]$ (for each $j$ such that $0 \leq j < Q$).
  • This procedure is called exactly once for each test case.

Constraints

  • $1 \leq N \leq 100\,000$
  • $1 \leq Q \leq 100\,000$
  • $1 \leq W[i] \leq 10^{9}$ for each $i$ such that $0 \leq i < N$
  • $1 \leq B[i] < A[i] \leq 10^{9}$ for each $i$ such that $0 \leq i < N$
  • $1 \leq E[j] \leq 10^{9}$ for each $j$ such that $0 \leq j < Q$

Subtasks

Subtask Score Additional Constraints
1 $6$ $Q \leq 5$; $N \leq 2000$; $W[i] = 1$ for each $i$ such that $0 \leq i < N$
2 $13$ $Q \leq 5$; $W[i] = i+1$ for each $i$ such that $0 \leq i < N$
3 $17$ $Q \leq 5$; $A[i] = 2$ and $B[i] = 1$ for each $i$ such that $0 \leq i < N$
4 $11$ $Q \leq 5$; $N \leq 2000$
5 $20$ $Q \leq 5$
6 $15$ $A[i] = 2$ and $B[i] = 1$ for each $i$ such that $0 \leq i < N$
7 $18$ No additional constraints.

Example

Consider the following call.

calculate_costs([15, 12, 2, 10, 21],
                [5, 4, 5, 6, 3],
                [1, 2, 2, 3, 2],
                [5, 9, 1])

In this example we have $N = 5$ artifacts and $Q = 3$ questions.

In the first question, $D = 5$. You can send artifacts $0$ and $3$ in one boat (since $|15 - 10| \leq 5$) and the remaining artifacts in separate boats. This yields the minimum cost of transporting all the artifacts, which is $1+4+5+3+3 = 16$.

In the second question, $D = 9$. You can send artifacts $0$ and $1$ in one boat (since $|15 - 12| \leq 9$) and send artifacts $2$ and $3$ in one boat (since $|2 - 10| \leq 9$). The remaining artifact can be sent in a separate boat. This yields the minimum cost of transporting all the artifacts, which is $1+2+2+3+3 = 11$.

In the final question, $D = 1$. You need to send each artifact in its own boat. This yields the minimum cost of transporting all the artifacts, which is $5+4+5+6+3 = 23$.

Hence, this procedure should return $[16, 11, 23]$.

Sample Grader

Input format:

N
W[0] A[0] B[0]
W[1] A[1] B[1]
...
W[N-1] A[N-1] B[N-1]
Q
E[0]
E[1]
...
E[Q-1]

Output format:

R[0]
R[1]
...
R[S-1]

Here, $S$ is the length of the array $R$ returned by calculate_costs.

Attachments
File name File size
nile.zip 2.4 KiB