#114 CrossingDots

[Shih-Yi]

CrossingDots
Given an n × n point lattice (intersection points of n consecutive horizontal and n consecutive vertical lines on common graph paper), where n > 2, cross out all the points by 2n − 2 straight lines without lifting your pen from the paper. You may cross the same point more than once, but you cannot redraw any portion of the same line. (A “greedy” solution for n = 4, shown in Figure 2.28, has seven lines instead of the six required by the puzzle.)