A classic pencil-and-paper game, the basis for the commercial game Battleship.

Also, a modern grid-logic puzzle based on the old game.

The usual rules for battleships are these:

• In a 10x10 grid (occasionally another size), there are 10 ships hidden, as follows:
• Four ships which each occupy a single space in the grid
• Three two-unit-long ships
• Two three-unit-long ships
• One four-unit-long ship
• In the grid, the one-unit ships are represented as circles, while the longer ones are represented by U-shapes at each end, and squares for the middle pieces. (So you don't know whether a square is part of a ship running horizontally or vertically.)
• The ships are all straight, each lying in a single row or column of the grid.
• These ships are all arranged in the grid so that no two ships touch, not even diagonally.
• For each row and each column, you are told how many units of ship pieces lie in it.
• Usually you are initially provided with the contents of a few squares -- just enough to make the solution unique.