In a column of a Sudoku puzzle, it may happen that a specific digit cannot be determined its exact position yet, but it is locked in a few cells as a candidate. That means the digit must be in one of a few cells. If these cells happen to fall in the same block, then this digit cannot be a candidate for other cells in that block.
Let's demonstrate this more clearly with an example.
3
9
1
7
9
1
3
7
9
1
3
7
9
3
9
2
5
2
5
7
5
9
5
6
7
3
5
6
9
7
8
9
7
8
1
5
1
9
1
5
1
3
9
1
3
5
7
9
3
8
2
3
7
1
3
8
1
2
3
1
3
5
8
2
5
2
3
5
2
3
5
1
3
5
7
1
2
7
3
5
7
8
7
8
1
3
5
8
1
2
8
1
9
1
8
9
1
3
1
3
8
5
6
5
6
1
7
8
1
7
8
1
8
In column C1 of this Sudoku puzzle, the digit 1 is a candidate for two yellow cells (R4,C1) and (R6,C1). In other words, the digit 1 in column C1 is locked in these two yellow cells, and one of them must be 1.
3
9
1
7
9
1
3
7
9
1
3
7
9
3
9
2
5
2
5
7
5
9
5
6
7
3
5
6
9
7
8
9
7
8
1
5
1
9
1
5
1
3
9
1
3
5
7
9
3
8
2
3
7
1
3
8
1
2
3
1
3
5
8
2
5
2
3
5
2
3
5
1
3
5
7
1
2
7
3
5
7
8
7
8
1
3
5
8
1
2
8
1
9
1
8
9
1
3
1
3
8
5
6
5
6
1
7
8
1
7
8
1
8
The two yellow cells (R4,C1) and (R6,C1) happen to fall in the same block B4.
3
9
1
7
9
1
3
7
9
1
3
7
9
3
9
2
5
2
5
7
5
9
5
6
7
3
5
6
9
7
8
9
7
8
1
5
1
9
1
5
1
3
9
1
3
5
7
9
3
8
2
3
7
1
3
8
1
2
3
1
3
5
8
2
5
2
3
5
2
3
5
1
3
5
7
1
2
7
3
5
7
8
7
8
1
3
5
8
1
2
8
1
9
1
8
9
1
3
1
3
8
5
6
5
6
1
7
8
1
7
8
1
8
Since one of the two yellow cells in block B4 must be 1 and each block cannot contain the same digit more than once, other cells (marked in orange) in block B4 cannot be 1. Therefore, the candidate digit 1's in these orange cells can be safely removed.