拓扑排序主要是为了解决一个工程能否顺利进行的问题，但有时我们还需要解决工程完成所需要的最短时间问题。我们如果对一个流程图获得最短时间，就需要分析它们的拓扑关系，并且找到当中的最关键流程，这个流程的时间就是最短时间。(Topological sorting is mainly to solve the problem of whether a project can be carried out smoothly, but sometimes we need to solve the shortest time problem needed for the completion of the project. If we get the shortest time for a flow chart, we need to analyze their topological relationships and find the most critical process, which is the shortest time.)