Also known as Hashiwokakero. Connect the numbered islands with straight bridges so every island ends up with exactly as many bridge-ends as its number shows — and the whole archipelago becomes one connected network.
Rules
Click an island, then click a neighbouring island to lay a bridge between them.
Click the same pair again to add a second, parallel bridge; a third click removes it.
Bridges run only horizontally or vertically, never cross another bridge, and never pass through another island.
Solved once every island's bridge-ends match its number and every island connects into one network.
Tips
An island numbered 1 has only one possible bridge — find its single neighbour and connect them immediately; islands in a corner are usually forced next.