Daily sudoku (X-wing training)
Level 3 sudoku puzzles on this website can be solved using advanced techniques, such as X-wing, XY-wing, XYZ-wing, skyscraper, kite, swordfish, and jellyfish, plus the techniques in level 1 and 2.
This page is especially for training with X-wing pattern only. For training with other patterns, check the links below.
Principles of X-wing:
Given two parallel and two perpendicular links, there are four intersection squares. Suppose, in the two parallel links, a number can only appear in the intersecton squares. Then, in the two perpendicular links, the number can only appear in the intersection squares as well. That is, this number cannot be in any other squares in the two perpendicular links.
Look at the example of X-wing pattern above. The first row, the last row, the 4th column, and the 7th column have four red intersection squares. In the first row, the number 4 can only appear in the two red squares. In the Last row, the same number 4 also can only appear in the red squares. Therefore, in the 4th and the 7th column, the number 4 must also be in the red squares. As a result, the number 4 cannot be in those two yellow squares.
Why? Because there must be two red squares with the number 4 in it, one in the 1st row, another the last row. If the number 4 appears in a yellow square, then the two red squares in the 7th column cannot contain 4. So the remaining two red squares have to take the number 4. But the remaining two red squares are in the same column. It's a violation of the sudoku rule that a number can only appear once in a column.
In the following example of X-wing pattern, the number 9 cannot be in the yellow squares.
