The problem of a pen was solved by the great mathematician Euler in 1736.

It has several principles:

(1) Every intersection point in the graph is an even point (the point where every even line intersects is called an even point), so this graph can be drawn with one stroke and can start from any point.

(2) There are a pair of singularities at the intersection of figures (the points where odd lines intersect are called singularities), and such figures can also be drawn with one stroke. But it must start at an odd point and end at another odd point.

(3) If there are more than one pair of singularities in a graph, the graph cannot be drawn with one stroke.

Therefore, if you want to draw a figure with one stroke, you must first determine the even and odd points in the figure, and then determine where to start drawing.

There are a pair of singularities in the picture, which can be drawn in one stroke. As shown in the figure: