void bfs(int start, int parent[], int dist[], int seen[], int visited[]) {
    std::queue <int> q;//建立數(shù)據(jù)隊(duì)列q
    int v;
    
    q.push(start);    //讓開(kāi)始序列入棧
    parent[start] = start;      // 開(kāi)始節(jié)點(diǎn)的父節(jié)點(diǎn)是開(kāi)始節(jié)點(diǎn)
    dist[start] = 0;            // 初始化距離向量為-1
    seen[start] = 1;       

    while(!q.empty()) {         //如果隊(duì)列非空
        v = q.front(); q.pop();   //令V是隊(duì)列的最前端，并將其出棧
        if(visited[v])          // 如果visited[v]=1, continue.
            continue;
        visited[v] = 1;         //否則令visited[v]=1
        std::cout << v << '\n';//輸出顯示當(dāng)前節(jié)點(diǎn)

        // 遍歷v的所有相鄰節(jié)點(diǎn)
        for(int i = 0; i < graph[v].size(); i++) {
            // 如果v的第i個(gè)相鄰節(jié)點(diǎn)的i并沒(méi)有訪問(wèn)過(guò)
            if(!visited[graph[v][i]]) {

                // 如果這個(gè)沒(méi)有訪問(wèn)過(guò)的節(jié)點(diǎn)沒(méi)有被看過(guò)
                if(!seen[graph[v][i]]) {
                //壓入棧，距離+1，設(shè)置父節(jié)點(diǎn)
                    q.push(graph[v][i]);
                    dist[graph[v][i]] = 1 + dist[v];
                    parent[graph[v][i]] = v;
                    // 如果已經(jīng)訪問(wèn)過(guò)，令seen=1.
                    seen[graph[v][i]] = 1;
                }
            }
            else {

                // 如果節(jié)點(diǎn)已經(jīng)被訪問(wèn)了，選擇距離最小的
                if(dist[v] + 1 < dist[graph[v][i]]) {
                    dist[graph[v][i]] = 1 + dist[graph[v][i]];
                    parent[graph[v][i]] = v;
                }
            }
        }
    }
}

int main() {

    int n = 8;          // 圖中的節(jié)點(diǎn)數(shù)
    graph = std::vector <std::vector <int> > (n);
    
    // 圖的鄰接表
    graph[0] = {1, 2};
    graph[1] = {0, 2, 3};
    graph[2] = {0, 1, 5, 6};
    graph[3] = {1, 2, 4};
    graph[4] = {3};
    graph[5] = {2};
    graph[6] = {2, 7};
    graph[7] = {6};

    /* - parent[i] = parent of 'i' in BFS traversal.
       - dist[i] = 從開(kāi)始到I節(jié)點(diǎn)的最短距離shortest distance of 'i' from 'start'.
       - If seen[i] == 1, 節(jié)點(diǎn)i已經(jīng)進(jìn)入過(guò)隊(duì)列'i' has entered the queue once
       - If visited[i] == 1, 節(jié)點(diǎn)i已經(jīng)進(jìn)入隊(duì)列，并且所有相鄰節(jié)點(diǎn)都已經(jīng)進(jìn)入過(guò)隊(duì)列
    */
    int parent[n+1], dist[n+1], seen[n+1], visited[n+1];
    memset(parent, -1, sizeof(parent));//父節(jié)點(diǎn)初始化為-1
    memset(dist, -1, sizeof(dist));//距離向量初始化為-1
    memset(seen, 0, sizeof(seen));
    memset(visited, 0, sizeof(visited));//seen用于判斷該節(jié)點(diǎn)是否訪問(wèn)過(guò)

    int start = 0;      // 開(kāi)始節(jié)點(diǎn)
    bfs(start, parent, dist, seen, visited);

    return 0;
}