9. Boundary-Seeded Multi-Source BFS | Sparse Grids (Codeforces Nearest Excluded Points)

Advancing our Multi-Source BFS toolkit. In this lecture, we solve Codeforces "Nearest Excluded Points" and introduce the "Inward Wave" technique.

Instead of searching outward from every point (which yields a Time Limit Exceeded verdict), we reverse the paradigm. We identify the "coastline" of our coordinate set—the boundary points directly adjacent to empty space—and seed them into our queue. From there, we run our Multi-Source BFS inward, allowing deep interior points to inherit the optimal exit coordinates from their parents in O(N log N) time. We also transition away from standard 2D arrays, utilizing std::set and std::map to handle sparse grids with massive coordinate bounds.

🔗 Problem Link: https://codeforces.com/contest/1651/problem/D

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Видео 9. Boundary-Seeded Multi-Source BFS | Sparse Grids (Codeforces Nearest Excluded Points) канала Yash Poonia