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首先需要说明的是拓扑排序是针对有向无环图来说的，有向无环图也就是DAG图，顾名思义就是整张图中的边都带有方向而且不存在环。
上面这张图就是一个简单的拓扑图，而下面这张图就不算是一个拓扑图
原因就在于下面这张图存在环B->C->D->B
现在我们知道什么是拓扑图了，下面我来说一下拓扑图对应的拓扑序列。
常见求拓扑序列的方法：
（1）从拓扑图中找到一个入度为0的点
（2）删除入度为0的点及与其相关联的边（相对应的边的另一端的点的入度会减一）
（3）在删边过程中遇到入度为0的点就加入队列
（4）重复上述操作，直到所有的点入度均变为0
容易发现拓扑图对应的拓扑序列不一定是唯一的，因为我们有可能同时找到多个入度为0的点，这个时候先删哪一个都可以，还以上面的图片为例求其拓扑序列：
其中一个拓扑序列对应A->C->B->D->E(下面这个图)
还有一个拓扑序列对应A->B->C->D->E(下面这个图)
如果一个图是连通的，我们可以通过入队的点数来判断图中是否存在环，有向无环图保证了图中的每一个点都能够入队一次，如果图中存在环，那么入队的点数一定小于图中所有点的点数和。
下面我给出拓扑序列核心代码：
void topsort() { queue<int> q; for(int i=1;i<=n;i++) if(!ru[i])
q.push(i);//入度为0的点入队 tt=0; while(q.size()) { int begin=q.top(); q.pop();
st[++tt]=begin;//记录拓扑序列 for(int i=h[begin];i!=-1;i=ne[i]) { int j=e[i];
ru[j]--; if(!ru[j]) q.push(j);//入度为0的点入队 } } }
