Hidden Single Technique, is another type of sole-candidate technique. Unlike Naked Single Technique, however in practice, it is not that obvious to determine if we can apply this technique. As we know, if there is only one candidate left in a cell, we can just place this number into the cell without hesitation. But is there any case that even if there are more than one candidates in a cell and we can still easily find the solution to this cell?
Consider the following example:
In Column 7, there are four candidates at cell [B7], but candidate 6 can only be found in this cell. In other word, if you don't place value 6 at cell [B7], no other cells in Column 7 can be filled with 6. Since every column must have all the digits from 1 to 9, we must set [B7] = 6.
Similarly, here's another example:
We can see that cell [B8] has the only candidate 7 in Row B. So [B8] is the only cell in Row B that can be placed with value 7. So we set [B8] = 7.
Of course, we can also find hidden singles in blocks:
In Block at [A1], candidate 8 only occurs cell [A2], so we just set [A2] = 8. And don't forget to update the candidates in related column, row and block.
Hidden Single Technique is a powerful supplement to Naked Single Technique. Many puzzles can be solved by using these two techniques alternately.
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