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玩数独,设计数独​

Play Sudoku, Design Sudoku

Method  to Solve a Hard Sudoku Puzzle  - Forcing Chain

2/17/2018

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Dr. Yaling Zheng

Whether you are traveling, taking a break, or recovering in hospitals, solving a number puzzle game (Sudoku Puzzle)  would be a perfect brain workout for you! 

When you solve a hard Sudoku puzzle, forcing chain method can help you determine exactly what number a certain empty square must hold.

Forcing chains work in the following way: Start on the square R4C4 (the square in the fourth row and the second column), and fill in one of the two candidates, 3 or 4. The following I am going to show no matter R4C4 is 4 or 6, R1C3 will always be number 4. This is exactly a great example of forcing chain. 
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If R4C4 is number 4, R1C4 has to be number 8 (step 1), and then R1C3 has to be number 4.

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If R4C4 Iis number 3, R4C7 has to be number 4 (step 1), and then R5C9 has to be number 3 (step 2), and then R3C9 has to be number 4 (step 3), and then R3C3 has to be number 8, and then R1C3 has to be number 4.

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Therefore, no matter what number R4C4 is, R1C3 will be number 4. This is a great example of using forcing chain to determine a certain number for an empty square.  Forcing chain is an advanced method for expert Sudoku solvers. It takes time and patience to find forcing chain patterns. And I only recommend you use this method if all other simpler methods do not work. 

Today's Sudoku (Difficulty Level: Hard)

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