8. Multi-Source Spanning Forests & Edge Pruning | BFS (Codeforces Police Stations)

Closing out Phase 2 with a masterclass in graph reduction. In this lecture, we tackle Codeforces "Police Stations" and explore how a Multi-Source BFS inherently generates a shortest-path spanning forest.

By seeding the queue with all police stations and expanding outward simultaneously, we naturally claim cities optimally. By tracking which edges are used during this "domain expansion," we isolate the absolute minimum roads required to keep the network connected to the stations. Any edge ignored by the BFS is mathematically redundant and safely deleted. We also break down why the distance constraint $d$ provided in the problem description is actually a clever competitive programming red herring.

🔗 Problem Link: https://codeforces.com/contest/796/problem/D

👋 𝐖𝐞𝐥𝐜𝐨𝐦𝐞 𝐭𝐨 𝐭𝐡𝐞 𝐜𝐡𝐚𝐧𝐧𝐞𝐥!⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣⁣ ⁣⁣⁣⁣⁣⁣
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Видео 8. Multi-Source Spanning Forests & Edge Pruning | BFS (Codeforces Police Stations) канала Yash Poonia