专题训练之双连通

桥和割点例题+讲解：hihocoder1183 http://hihocoder.com/problemset/problem/1183

  1 #include<cstdio>
  2 #include<cstring>
  3 #include<algorithm>
  4 #include<vector>
  5 #include<set>
  6 using namespace std;
  7 const int maxn=1005;
  8 const int maxm=200010;
  9 struct edge{
 10     int to,nxt;
 11     bool cut;
 12 }edge[maxm*2];
 13 int head[maxn],tot;
 14 int low[maxn],dfn[maxn];
 15 int index,n,bridge;
 16 set<int>st;
 17 bool cut[maxn];
 18 
 19 void addedge(int u,int v)
 20 {
 21     edge[tot].to=v;
 22     edge[tot].nxt=head[u];
 23     edge[tot].cut=false;
 24     head[u]=tot++;
 25 }
 26 
 27 void tarjan(int u,int pre)
 28 {
 29     low[u]=dfn[u]=++index;
 30     int son=0;
 31     for ( int i=head[u];i!=-1;i=edge[i].nxt ) {
 32         int v=edge[i].to;
 33         if ( v==pre ) continue;
 34         if ( !dfn[v] ) {
 35             son++;
 36             tarjan(v,u);
 37             low[u]=min(low[u],low[v]);
 38             if ( low[v]>dfn[u] ) {
 39                 bridge++;
 40                 edge[i].cut=true;
 41                 edge[i^1].cut=true;
 42             }
 43             if ( low[v]>=dfn[u] && u!=pre ) {
 44                 st.insert(u);
 45                 cut[u]=true;
 46             }
 47         }
 48         else if ( low[u]>dfn[v] ) low[u]=dfn[v];
 49     }
 50     if ( u==pre && son>1 ) {
 51         cut[u]=true;
 52         st.insert(u);
 53     }
 54 }
 55 
 56 void solve()
 57 {
 58     memset(low,0,sizeof(low));
 59     memset(dfn,0,sizeof(dfn));
 60     memset(cut,false,sizeof(cut));
 61     index=bridge=0;
 62     st.clear();
 63     for ( int i=1;i<=n;i++ ) {
 64         if ( !dfn[i] ) tarjan(i,i);
 65     }
 66     set<int>::iterator it;
 67     if ( st.size()==0 ) printf("Null\n");
 68     else {
 69         for ( it=st.begin();it!=st.end();it++ ) {
 70             if ( it!=st.begin() ) printf(" ");
 71             printf("%d",*it);
 72         }
 73         printf("\n");
 74     }
 75     vector<pair<int,int> >ans;
 76     for ( int i=1;i<=n;i++ ) {
 77         for ( int j=head[i];j!=-1;j=edge[j].nxt ) {
 78             if ( edge[j].cut && edge[j].to>i ) ans.push_back(make_pair(i,edge[j].to));
 79         }
 80     }
 81     sort(ans.begin(),ans.end());
 82     for ( int i=0;i<ans.size();i++ ) {
 83         printf("%d %d\n",ans[i].first,ans[i].second);
 84     }
 85 }
 86 
 87 void init()
 88 {
 89     tot=0;
 90     memset(head,-1,sizeof(head));
 91 }
 92 
 93 int main()
 94 {
 95     int m,i,j,k,x,y,z;
 96     while ( scanf("%d%d",&n,&m)!=EOF ) {
 97         init();
 98         while ( m-- ) {
 99             scanf("%d%d",&x,&y);
100             addedge(x,y);
101             addedge(y,x);
102         }
103         solve();
104     }
105 }

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