Hidden Single
beginnerWhen a digit appears as the only candidate in one cell within a Region, it’s called a Hidden Single. This digit must be placed in that cell.
Hidden Single is a basic strategy in Sudoku.
It occurs when, for a given digit in a row, column, or 3x3 box, only one cell remains as a valid candidate. The cell may have other candidates, but in the specific row, column, or 3x3 box, the digit can only go in that cell.
See the examples below for clarification.
Example 1
In row B, 7 is a Hidden Single: the digit 7 can only go in cell B6.
7 cannot be placed in B1, B2, or B3 because of the 7 in C2. Additionally, 7 cannot be placed in B7 due to the 7 in A9. This leaves only one possible location for 7 in row B — B6.
Example 2
In row E, 6 is a Hidden Single: the digit 6 can only go in cell E6. Can you figure out why?
Additionally, on the same board, 6 is also a Hidden Single in column 6.
Example 3
The following example demonstrates how the Hidden Single strategy applies in the world of variants.
In row B, B3 is a Hidden Single with the digit 4. At first glance, it might seem that 4 could also go in B2. However, since this is a board with a palindrome, the candidates in B2 are restricted by the possible candidates in cell F9 (these two cells must contain the same digit). Since 4 cannot be placed in F9, it can be eliminated from B2, leaving B3 as the only valid location for the 4. Therefore, B3 is a Hidden Single with the value 4.
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