拓扑排序

L← Empty list that will contain the sorted elements
S ← Set of all nodes with no incoming edges
while S is non-empty do
    remove a node n from S
    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 S
if graph has edges then
    return error (graph has at least onecycle)
else
    return L (a topologically sortedorder)

bool toposort() {
  q = new queue();
  for (i = 0; i < n; i++)
    if (in_deg[i] == 0) q.push(i);
  ans = new vector();
  while (!q.empty()) {
    u = q.pop();
    ans.push_back(u);
    for each edge(u, v) {
      if (--in_deg[v] == 0) q.push(v);
    }
  }
  if (ans.size() == n) {
    for (i = 0; i < n; i++)
      std::cout << ans[i] << std::endl;
    return true;
  } else {
    return false;
  }
}

// C++ Version
vector<int> G[MAXN];  // vector 实现的邻接表
int c[MAXN];          // 标志数组
vector<int> topo;     // 拓扑排序后的节点

bool dfs(int u) {
  c[u] = -1;
  for (int v : G[u]) {
    if (c[v] < 0)
      return false;
    else if (!c[v])
      if (!dfs(v)) return false;
  }
  c[u] = 1;
  topo.push_back(u);
  return true;
}

bool toposort() {
  topo.clear();
  memset(c, 0, sizeof(c));
  for (int u = 0; u < n; u++)
    if (!c[u])
      if (!dfs(u)) return false;
  reverse(topo.begin(), topo.end());
  return true;
}

# Python Version
G = [] * MAXN
c = [0] * MAXN
topo = []

def dfs(u):
    c[u] = -1
    for v in G[u]:
        if c[v] < 0:
            return False
        elif c[v] == False:
            if dfs(v) == False:
                return False
    c[u] = 1
    topo.append(u)
    return True

def toposort():
    topo = []
    while u < n:
        if c[u] == 0:
            if dfs(u) == False:
                return False
        u = u + 1
    topo.reverse()
    return True