Star Battle

The goal is to use logic to determine where the stars are in the supplied grid. Only two stars can appear in each row, column and region. Also, no two stars may touch, even diagonally.

Sample Puzzle

Solution

By looking at which stars are already in the different rows, columns and regions you can deduce where other stars may go, or at least eliminate spots where you know that they cant go. By examining multiple rows or columns at the same time, you can figure out where all the stars should go.

Techniques and Strategies

Examine this shape made up of 5 squares. Because it must contain two stars and because no two stars can touch, even diagonally, we know that the star must be in the indicated place and we can mark off all 8 sourounding squares.

Examine this shape made up of 6 squares. If the star went in the middle square, then there would be no other legal place to put the second star. Therefore this middle spot cannot contain a star.

Examine this horseshoe shape. Every possible combinations of two stars that it could contain would be adjacent to the square in the other shape, so it cannot contain a star.

Look at this small 4 square shape. It must contain two stars, which means that the other squares in that row cannot. We can also use the "External Elimination" technique to mark off some additional squares.

Another common strategy is to find a spot where you know that a star must go into either one of two or three spaces. Then use pencil marks to place a trial star in a square and use additional pencil marks to indicate what this forces. If you get a contradiction or reach an impossible situation then you know that our intitial guess was incorrect and it can be eliminated. If you dont reach a contradiction, it doesn't necessarily mean that your guess was correct. Back up and try the other candidates and see if they work as well. Sometimes you will have to make several guesses to find some additional clues.