Every point that can be drawn in one stroke on the diagram, except the starting point and the ending point, should be connected with even lines. This kind of point is called even point. Points connected by odd lines are called odd points. There should be no odd points in a graph that can be drawn with one stroke except the starting point and the ending point.

The most famous mathematical problem is the seven-bridge problem (Euler solution). The concept of a stroke is to discuss whether a figure can be drawn with one stroke. Any endpoint in a graph is divided into singular points and even points according to the number of connecting lines. Only figures with even points and figures with only two singularities can draw strokes. A graph with only even points has no starting point, and only two singularities must start from one point to another. In any graph, singularities appear in pairs, and there is no graph with odd points.

1. Any connected graph composed of even points can be drawn with one stroke. When drawing, you can start from any even point, and finally you can finish drawing with this point as the end point.

3. Any connected graph with only two singularities (the rest are even points) can be drawn with one stroke. When drawing, one singularity must be the starting point and the other singularity must be the end point.

[14] Other situations cannot be brushed aside. Divide the odd number by two, and you can work out how many strokes you need to draw this picture. )

There are four strange points in this picture, so you can't draw it with one stroke.