定义：
Topological Sorting is a method of arranging the vertices in a directed acyclic graph (DAG有向无环图) as a sequence, such that for every directed edge(u,v), 即u-> v, vertex u comes before v in the ordering
注意：
A topological ordering is possible if and only if the graph has no directed cyclesTopological orderings are NOT unique
应用场景：
build systems: a program cannot be built unless its dependencies are first built pre-requisite problems: a course cannot be selected unless its pre-requisite courses are taken dependency problemstask schedule
时间复杂度:
O(V+E)
Topological Sort(){ 1. Call DFS to compute finish time f[v] for each vertex 2. At each vertex is finished, insert it onto the front of a linked list 3. Return the linked list of vertices }
内部原理：
比如从任意一个vertex出发（比如，随便选个NodeH）， 来DFS搜索
从NodeH出发，DFS可以走 NodeH-> NodeJ
从NodeJ出发， DFS可以 NodeJ -> NodeM
发现NodeM后面没有元素了， 我们把NodeM放到Stack里面（基于Stack的FILO特性）
从NodeM往回走，到NodeJ， 从NodeJ出发，DFS可以NodeJ-> NodeL
发现NodeL后面没有元素了， 我们把NodeL放到Stack里面
以此类推，DFS走完整个有向无环图，并用Stack保证了u-> v, vertex u comes before v in the ordering
综上，
1. 从有向无环图的任意点出发
2. 进行DFS搜索， 直到搜到当前元素已经"nowhere left to go"了
3. 将该元素放到Stack里，并将该元素mark为visited
4. 从该元素backtrack，继续DFS
转载于:https://www.cnblogs.com/liuliu5151/p/10487164.html
