#include 
#include 
#include 
#include 

using namespace std;

void bfs(const vector>& graph, int start) {
    int n = graph.size();
    vector visited(n, false);
    queue q;

    visited[start] = true;
    q.push(start);

    while (!q.empty()) {
        int u = q.front();
        q.pop();
        cout << u << " ";  // 访问当前节点

        // 遍历所有邻接点
        for (int v : graph[u]) {
            if (!visited[v]) {
                visited[v] = true;
                q.push(v);
            }
        }
    }
    cout << endl;
}

// 示例用法
int main() {
    // 构建 4 个节点的有向图：0→1, 0→2, 1→3, 2→3
    vector> graph(4);
    graph[0] = {1, 2};
    graph[1] = {3};
    graph[2] = {3};
    // graph[3] 为空

    bfs(graph, 0); // 输出：0 1 2 3
    return 0;
}