"One trick" is an ancient problem, which Europeans call "the postman problem". Facing the complicated city streets, the postman needs to deliver the mail to the customers scattered in the streets. In order to avoid going the wrong way, it is necessary to plan the route reasonably before departure, and the knowledge needed is "one trick".

The law of stroke:

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.

2. 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.

Extended data

1736, Euler confirmed that there was no solution to the seven-bridge problem. At the same time, he published "a theorem": if a graph wants to be completed in one stroke, it must meet two conditions:

1, graphics are connected;

2. The number of singularities (points connected with odd edges) in the graph is 0 or 2;

Euler's research initiated a new branch of mathematics ―― graph and geometric topology.