Competitive Programming

/**
 * Author: Dean Zhu
 * Date: 2018-11-15
 * License: CC0
 * Description: Finds all articulation points and bridges of a undirected graph.
 * Time: O(E + V)
 * Status:
 */

void ArticulationPointandBridges(vector<vi>& adj, vector<Edge> E, vi& articulation, vi& bridges) {
    int n = sz(adj), m = sz(E);
    articulation = vi(n, -1);
    bridges = vi(m, -1);
    vi dfs_low(n, -1), dfs_num(n ,-1), parent(n, -1);
    articulation = vi(n, 0);
    bridges = vi(m, 0);
    int dfsroot, rootc, cnt = 0;
    function<void(int)> ABdfs = [&](int u) {
        dfs_num[u] = dfs_low[u] = cnt++;
        for(int t : adj[u]){
            int v = E[t].u^E[t].v^u;
            if(dfs_num[v] == -1){
                if(u == dfsroot) rootc++;
                parent[v] = t;
                ABdfs(v);
                dfs_low[u] = min(dfs_low[u], dfs_low[v]);
                if(dfs_low[v] >= dfs_num[u])articulation[u] = 1;
                if(dfs_low[v] > dfs_num[u]) bridges[t] = 1;
            } else if (t != parent[u]){
                dfs_low[u] = min(dfs_low[u], dfs_num[v]);
            }
        }
    };
    FOR(i, 0, n) {
        if(dfs_num[i] == -1) {
            rootc = 0, dfsroot = i;
            ABdfs(i);
            if (rootc <= 1) articulation[i] = 0;
        }
    }
}

int main() {
    int n,m;
    while(scanf("%d",&n)==1)
    {
        vector<Edge> E;
        vector<vector<int>> adj(n+1);
        vi articulation;
        vi bridge;
        bool connected[n+1][n+1];
        memset(connected,false, sizeof(connected));
        for(int i=0; i<n;i++)
        {
            int x,y; scanf("%d (%d)",&x,&m);
            for(int j=0; j<m;j++)
            {
                scanf("%d",&y);
                if(connected[x][y] || connected[y][x] ) continue;
                connected[x][y] = true;
                E.push_back({x,y});
                adj[x].push_back(E.size()-1);
                adj[y].push_back(E.size()-1);
            }
        }
        ArticulationPointandBridges(adj,E,articulation,bridge);

        int cnt = 0;
        vector<Edge> resu;

        for(unsigned i=0;i<bridge.size();i++)
            if(bridge[i]==1) { cnt++; resu.push_back(reverse(E[i]));}
        printf("%d critical links\n",cnt);
        sort(resu.begin(),resu.end(),cmp);
        for(int i=0; i<cnt;i++)   printf("%d - %d\n", resu[i].u,resu[i].v);
        cin.ignore();  printf("\n"); }
    return 0;
}