Blacken some cells in the diagram so that in the remaining cells no number occurs more than once per row or column. All unblackened cells must be connected. Two blackened cells may not be adjacent, though may touch by a corner.
Grid control: Click the mouse button at the selected cell to change the color of the cell. Second click will put auxiliary circle, which doesn't affect correctness of the solution, but helps visually. You can also drag the content of one cell to another with the mouse. More details about grid control are at the help page.