说明：  这个说明倒不是说明函数的功能，只是声明一下函数中出现的问题。 shortest()函数没有问题，在secshortest中我希望只得到一条次短路（或几条roadcost()值相同的次短路），结果却出来好几条，我仔细检查了一下程序，应该没有什么问题。 因此在使用时，要得到次短路，只能用roadcost()函数分别求一下各条路的权重和，找到最小的。 有些奇怪
(Note that this is not the function of function, but a statement about the problems in function. shortest () function there is no problem, I hope that in secshortest only a short time (or several roadcost () times the value of the same short-circuit), the result is several out, I carefully examined the process, it should be no problem. Therefore, in use, to be at a short circuit can only be used roadcost () function, respectively, for some of the weight of the road and find the smallest. Some strange)