XY-Wing
Look for a cell with exactly two candidates, XY. If there are two cells, XZ and YZ, that both share a unit with XY, then we can be certain that either XZ or YZ will contain the value Z. So, we can remove the candidate Z from the intersection of XZ and YZ.
Consider the sudoku below.
The cell r2c6 can take the role of XY, where X = 4 and Y = 5. Subsequently,
The cell r2c4 shares the common candidate 4, and can take the role of XZ, where Z = 1.
The cell r9c6 then exactly fits the role of YZ with the candidates 5 and 1. Therefore, either
r2c4 or r9c6 must contain the digit 1. So we can remove the candidate 1 from
the intersection of r2c4 and r9c6.
Note: The intersection of two cells is defined as all cells that share a unit with both cells.
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Last Updated: 6/21/2018 5:33:58 AM CST