说明：  在这个函数的编制中存在一个细节，当任取一个已标号未检查的点的时候，我取的最靠前的点。如果加进随机选取的语句，每次运行程序可能会出现不同的最大流结果。
(There is a detail in the preparation of this function, and when I take a point that has not been checked, I take the closest point. If you add a randomly selected statement, you may run a different maximum flow result each time you run the program.)