Rules of Hashiwokakero

Draw horizontal and vertical lines (bridges) connecting circles (islands) such that:

1. Between any two islands, there are at most two such bridges connecting them.
2. Bridges may not cross each other or pass over an island.
3. A number on an island indicates how many bridges connect it to some other island.
4. All islands are bridge-connected; i.e. it is possible to move from any island to any other island by only moving across bridges and islands.

Below is an example and its answer.