Plankway

Full island: when an island has all its bridges

This is the technique you will use more than any other, often without noticing. When an island already has as many bridges as its number says, it is finished. Nothing else may be built on it, and that tells you something about every one of its neighbours.

The idea

Count the bridges that are certain on an island. If that count equals the number, every other connection from that island is impossible: no new bridge, and no second bridge on a connection that already has one.

ABCDE1234521321
The 2 in C3 already has a double bridge to the left. It is full, so the three other connections are ruled out.

Ruling out connections may sound like a small gain, but look at the neighbours. The 1 in C1 and the 1 in C5 just lost one of their options. If they have only one option left, their bridge is now certain.

The 1 is the classic case

An island with a 1 is full as soon as it has a single bridge. In the figure below, the 1 in A1 is connected to the 3 on the right, so its connection downwards is gone. The 2 in A3 now has to find both its bridges elsewhere.

ABCD123132
The 1 in A1 has its bridge. The connection to the 2 below it is ruled out.

How to use it while playing

  • Every time you place a bridge, check both islands it touches. Is either of them full now?
  • In Plankway a full island raises a yellow flag. Use it as a reminder: a flag means "nothing more here", so look at its neighbours.
  • A red ring with "!" means an island has too many bridges. That is always a mistake; tap a bridge to remove it.

Full island works hand in hand with every bridge counts: one rules connections out, the other fills them in. Most of a Monday puzzle is just these two, back and forth.

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