拓扑排序
对一个有向无环图(Directed Acyclic Graph简称DAG)G进行拓扑排序，是将G中所有顶点排成一个线性序列，使得图中任意一对顶点u和v，若边(u,v)∈E(G)，则u在线性序列中出现在v之前。通常，这样的线性序列称为满足拓扑次序(Topological Order)的序列，简称拓扑序列。简单的说，由某个集合上的一个偏序得到该集合上的一个全序，这个操作称之为拓扑排序(Topological sort
A topological sort for a Directed Acyclic Graph (DAG) G is a linear sequence of all vertices in G, which makes any pair of vertices u and V in the graph, and if the edge (U, V) E (G) appears in the linear sequence, it appears before the linear sequence. Usually, such a linear sequence is called a sequence satisfying topological order (Topological Order), or topological sequence. Simply speaking, a complete order on the set is obtained from a partial order on a set. This operation is called topological sorting.)