The Chase
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Bessie is trying to evade the farmers. The farmers own \(N\)
(\(2 \le N \le 5 \cdot 10^5\)) farms with a one way road between the \(i\)-th farm
and the \(a_i\)-th farm (\(1 \le i \le N\), \(a_i \neq i\)). There are \(F\)
(\(1 \le F \le N\)) farmers and the \(i\)-th farmer is initially stationed at farm
\(s_i\) (\(1 \le s_i \le N\), all \(s_i\) unique). At each time step, every farmer
takes the road at their current farm and moves to the next. Bessie gets caught
if she is ever located at the same farm as any farmer.
Suppose Bessie starts at some farm \(b\). At each time step, she has two options:
she can either choose to rest (stay at her current farm) or take the road and
move to the next farm. If she chooses to move, she moves simultaneously with the
farmers. Bessie moves so that she is never caught by any farmer in a finite
number of timesteps.
For each starting farm \(b\) (\(1 \le b \le N\)), find the maximum number of times
Bessie can choose the rest option if she starts at farm \(b\).
Problem credits: Alex Liang
SCORING
- Input 2: \(N \le 50\)
- Inputs 3-10: \(N \le 2000\)
- Inputs 11-20: No additional constraints.
Problem credits: Alex Liang
The first line contains \(N\) and \(F\), the number of farms and the number of
farmers.
The second line contains \(a_1 \ldots a_N\), the one-way roads going out of each
farm.
The third line contains \(s_1 \ldots s_F\), the starting locations of each farmer.
Output \(N\) lines, the \(b\)-th of which must consist of a single integer denoting
the maximum number of times Bessie can choose the rest option if she starts at
farm \(b\). If there is no way for Bessie to ensure she is never caught after a
finite number of timesteps, then output \(-1\). If Bessie can rest an infinite
number of times, then output \(-2\).
4 1
2 1 4 3
1-1
0
-2
-2- Farm 1: If Bessie starts at a farm with a farmer, then she is caught immediately, and you should output \(-1\).
- Farm 2: Bessie must choose to move at every timestep to avoid being caught by the farmer who starts at farm \(1\).
- Farms 3-4: Bessie can rest an infinite number of times without being caught.
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출처 올림피아드 > USACO > 2025-2026 > Second Contest > Gold
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