Hidden Island Expansion

Sometimes an island is too large to fit in a given area. In such scenarios, we need to look for possible expansion paths outside the apparent constraints. If an expansion path is found, then the adjacent cell in the opposite direction, which cannot belong to any island, is shaded as part of a wall.

Let's illustrate with the following example:

Here, the island of '12' cannot fit in the area at the top and must therefore expand downwards. The cell to the left of this new island cell cannot belong to any island and is therefore shaded.