Merge pull request #2649 from zhb2000/patch-1

## DFS 算法

```cpp

// dfs 版本

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 = 0; v <= n; v++)

    if (G[u][v]) {

  for (int v : G[u]) {

    if (c[v] < 0)

      return false;

    else if (!c[v])

        dfs(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++)

  for (int u = 0; u < n; u++)

    if (!c[u])

      if (!dfs(u)) return false;

  reverse(topo.begin(), topo.end());