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For Level 4 sudoku puzzles on this page, it needs to use the divide-and-conquer technique once, plus techniques from lower levels. The divide-and-conquer technique can be applied when one of the following situations occurs:
- Forcing chain.
There are only two numbers that can appear in a square. In this case, start from each number in the square to solve the puzzle for a few steps. Some numbers may be found impossible in certain squares.
- Coloring.
There is a number that can only appear in two squares in a link. In this case, start from this number in each square to solve the puzzle for a few steps. Some numbers may be found impossible in certain squares.
Some examples for the divide-and-conquer technique are explained at the following link.
In the following link, the divide-and-conquer technique has to be used for 17 times to solve the sudoku puzzle.
The divide-and-conquer technique is only used when combining level 1, 2, and 3 techniques cannot solve the sudoku puzzle completely.
For level 3 techniques, check links below.