用栈实现 DFS 遍历无向图，横线处（标记并入栈邻居）应填入：
for (int neighbor : graph[node]) {
if (!visited[neighbor]) {
________ // visited[neighbor] = true;
________ // s.push(neighbor);
}
}
- A. visited[neighbor]=true; s.push(neighbor-1);
- B. visited[neighbor]=true; s.push(neighbor+1);
- C. visited[neighbor]=false; s.push(neighbor);
- D. visited[neighbor]=true; s.push(neighbor);
正确答案：D