Bridges #515 · 9×9

• 9×9grid
• 15islands
• 19bridges
• 4double routes

Bridges #515 is a 9×9 grid at medium level with 15 islands. The solution lays 19 bridges along 15 routes, and 4 of those routes carry a double bridge. The largest number on the map is 6.

You can start without any guessing: 4 islands have a number equal to or one below twice the neighbours they can see, so every one of those neighbours gets at least one bridge.

5 islands carry a 1. Two of them may never be linked to each other, because that pair would be cut off from the rest of the map.

Medium grids also need the isolation rule: rule out any bridge that would close a group of islands off from the rest of the map.