拓扑排序（来自维基百科）：

在图论中，有向无环图（DAG）的拓扑排序或拓扑排序是其节点的线性排序，其中每个节点先于其具有出站边缘的所有节点出现。每个DAG具有一种或多种拓扑类型。

伪代码：

L ← Empty list where we put the sorted elements
Q ← Set of all nodes with no incoming edges
while Q is non-empty do
    remove a node n from Q
    insert n into L
    for each node m with an edge e from n to m do
        remove edge e from the graph
        if m has no other incoming edges then
            insert m into Q
if graph has edges then
    output error message (graph has a cycle)
else 
    output message (proposed topologically sorted order: L)