Puzzle 4 – Battleship Sudoku
Invented by Thomas Snyder, Battleship Sudoku was one of the early puzzle types I discovered a few years back. The givens in this puzzle form an anchor which was later asked as the final question at Yubotu Island.
As with most Battleships, you start by locating the longest ship, here it is the 6587-battleship. Scanning column to column, row to row, you can see that there is only one available row for the Battleship. Conveniently, only one exact location.
The next step is to look at the 8th column. There are 4 vacant squares for three ships. If the lone cell (R7C8, shaded red in the diagram below) is occupied, it will eliminate 2 cells in the 6th row which will force all 4 remaining cells to be occupied.Should this happen, the 4th column cannot take 3 ship parts (it can only take 2), so we can deduce that R7C8 will be empty, and that the 3 remaining cells of the 8th column will be occupied by a 3-cell long destroyer.