X-wing strategy for solving sudoku
Advanced strategies
X-wing
To carry out a X-wing, we must find 2 lines or 2 columns where there is twice, and only two, the same candidate and which these twice candidates are aligned to form a square (or a cross according to the name of the strategy). All the candidates being in the same lines and columns of the square have then to be eliminated.
Example
First of all we isolate the candidates from a grid, here we will choose the "4".
We notice that the first line and the last line contain each one two "4" and that these "4" are aligned of a line on the other. We are well in the presence of a "X-wing".
Indeed, if one from the "4" of the first line is the good, that which is opposite for him is also good.
And the reciprocal is true in the other direction.
We notice that the figure formed by the relation between each opposite corner forms a "X".
Whatever the good solution of "X" [ in top on the left and bottom on the right... or top on the right and bottom on the left ], we are sure that "4" which is in the columns in clear green are false, we must eliminate them.
Strategies list
Elimination of candidates by interaction in the same area
Elimination of candidates by interaction in several areas
Elimination of candidates by interaction in all the puzzle
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