tarjan算法——割边（桥）

 1 int vis[maxn],dfn[maxn],low[maxn];
 2 int n,m,a[maxn][maxn],lay;
 3 #define mp make_pair
 4 #define fi first
 5 #define se second
 6 vector<pair<int,int> >ans;
 7 void tarjan(int u,int father) {
 8     low[u]=dfn[u]=++lay;
 9     vis[u]=1;
10     for(int v=0;v<n;v++) {
11         if(v==father) continue;
12         if(a[u][v]&&!vis[v]) {
13             tarjan(v,u);
14             low[u]=min(low[u],low[v]);
15             if(dfn[u]<low[v])
16                 ans.push_back(mp(min(u,v),max(u,v)));
17         }
18         else if(a[u][v]) low[u]=min(dfn[v],low[u]);
19     }
20 }