Depth First Search 遍历功能

该函数采用当前节点索引的参数，邻接列表（在此示例中存储在向量的向量中）和布尔向量来跟踪已访问的节点。

void dfs(int node, vector<vector<int>>* graph, vector<bool>* visited) {
    // check whether node has been visited before
    if((*visited)[node])
        return;

    // set as visited to avoid visiting the same node twice
    (*visited)[node] = true;

    // perform some action here
    cout << node;

    // traverse to the adjacent nodes in depth-first manner
    for(int i = 0; i < (*graph)[node].size(); ++i)
        dfs((*graph)[node][i], graph, visited);
}