With this strategy, the player finds two or three cells with the same pattern breaker but these are not necessarily connected by row, column and/or block. The 1 can thus be removed as a candidate to either of them. This means that the orange cells contain a hidden naked pair formed by the 6 and the 5. However, the 5 is not a candidate for any other position within that block. The player knows that one orange cell will contain a 6 while the other will be either a 1 or a 5. In this example, the pair 1/5 forms the deadly pattern while the 6 works as the pattern breaker. If out of the base pair, one digit is not a candidate to any other cell within that row, column or block, then the second digit of the pair becomes invalid in those two cells. The player knows that these two cells will be filled with the extra candidate plus one of the digits of the deadly pattern. A combination of the two strategies is also possible.Īs with the Type 2, the player finds the same pattern breaker in two different cells that share a column or row, but the Type 4 technique, these also need to share the same block. The Type 4 technique is only a variation of Type 2. They can then be eliminated from the remaining cells in that same column. In short, the player knows for sure the 1 and the 3 will be positioned in one of those three cells. If 1 is the pattern breaker then its solution will be the 3. Thus, if the 3 is the pattern breaker the solution to the orange cell will be 1. In their column, there is also a cell containing only this pair as candidates (highlighted in orange). The player knows that either one or the other will be positioned in one of those two cells. The numbers 3 and 8 are the pattern breakers in this example. When available, it is possible to eliminate the pattern breakers as candidates to other cells in the areas shared by the pattern and that specific cell. It must also be connected to the base cells. The player must then search for a cell that contains only the pair of pattern breakers as candidates. Yet, those cells will be for certain the solution to one of them. As with Type 2, two aligned cells of the pattern contain extra candidates but now these digits are not similar. This technique combines the basic rectangular pattern with the concept of locked subsets. The digits circled in red can be eliminated. Therefore, it cannot be a candidate for any other cell within that group or row. As this digit will have to break the pattern, the player knows for certain that its solution is within those 2 cells. Numbers 7 and 6 form the deadly pattern in this puzzle, with number 4 working as the extra digit. If these cells share the same row, column and/or group, it is possible to remove the digit as a candidate to any other cell in those areas. In this pattern, the player finds the same setting as before, but now two cells contain the same extra candidate. The Type 2 strategy follows the same assumptions as of Type 1 but instead of providing a solution, it helps to reduce the number of candidates. The numbers 2 and 3 can be eliminated as candidates for that cell. These are the digits that will break the deadly pattern. Nevertheless, one of the cells in this rectangle has two extra candidates, the 1 and the 8. Thus, they could potentially create a double solution situation as the position of the 2 or the 3 is indifferent. In this example, the four cells containing the pair 2/3 form a unique rectangle as they are not affected by any other cell in the grid. This number is necessary to break the deadly pattern and it is the solution to that cell. However, a well-designed grid will always ensure that an extra candidate is also possible in at least one of the cells of the rectangle. This is a deadly pattern that results in a double solution for the puzzle. They are grouped in parallel sets of two, sharing the same columns, rows, and blocks. This strategy applies when the player finds four cells with the same pair of candidates facing each other. Thus, when facing a disposition that may potentially lead to multiple solutions, they reveal the only possible course of action. This set of strategies assumes that Sudoku puzzles cannot have more than one solution. In easier puzzles, these strategies have a high chance of revealing the solution for one or more cells, while in harder levels they can either provide a solution or help to reduce the number of candidates to several cells. The Unique Rectangle set of strategies comprises useful techniques that are relevant throughout all the difficulty levels of Sudoku.
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