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deleteto remove the content of a cell.
The Sudoku is always verified using the same procedure regardless of which button is used, but if you only want to check the correctness without revealing any information about unknown cells you should use the validate button.
The first check is to verify that no digit occur more than once in each row, column and 3x3 square. If it turns out that some digits are wrongly placed they are marked with red colour and the row, column or 3x3 square where the error was found is marked with red background.
The next check is to search for a solution. This usually takes less than a second but for some difficult Sudokus it can sometimes take a little longer. If there is no solution an error message is displayed that says the Sudoku doesn't have a solution. This probably means that you have made a mistake, but you might want to validate the Sudoku without your own digits just to make sure the Sudoku has been constructed correctly.
Use the button marked solution to show a complete solution to the Sudoku puzzle. The tool never shows more than one solution even if there are multiple ways to solve it. A well-designed Sudoku puzzle only have one possible solution.
If you only want to know the digits for a few cells without revealing the whole solution you can use the reveal x button. Before pressing the button you have to first mark the cells that you want to reveal with the letter x.