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Editorial Team
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Asked: 2026-06-01T02:31:58+00:00

Consider this problem: There’s a square grid defined, each tile being either passable (1)

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Consider this problem:

There’s a square grid defined, each tile being either passable (1) or impassable (0).
At first, we have a simply connected space in the grid with an impassable border, like this:

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We then start placing impassable obstacles of various dimensions (e.g. 1×1, 2×2,..) into the passable space. After each obstacle is placed, we need to test whether the remaining passable space is still connected (i.e. make sure we didn’t split the passable space in two or more disconnected spaces). Tiles are connected diagonally, too.

The point is that after every obstacle placement, every remaining passable tile has a path that connects it to EVERY other remaining passable tile.

I’m aware of the possibility of searching for paths between possibly disconnected points, but I’m afraid that might be too inefficient. What I’m interested in is doing this testing as fast as possible.

Thanks for any help!

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1 Answer

  1. Editorial Team

    Implement a flood fill algorithm. As a side effect of performing the fill, count the number of squares filled. After placing your obstacles perform another flood fill starting from any open square and compare the number of filled squares to the original number minus the number of squares placed as obstacles. If they are not the same, you have disconnected regions.

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