Editorial for COCI '20 Contest 2 #2 Odašiljači

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First, let's discuss how to check for some fixed radius r if all antennas can communicate. Two antennas can communicate directly if their distance is at most 2r. We can build a graph over antennas, and add edges for all pairs of antennas that are closer than 2r. Then, we need to check if the graph is connected, which can be done for example with DFS.

The optimal r, that is the minimal r for which the constructed graph is connected, can be found using binary search. For the upper bound, we can put for example 10^9. The complexity is \mathcal O(n^2 \log_2 10^9).


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