kuangbin专题 专题九 连通图 Critical Links UVA - 796

题目链接：https://vjudge.net/problem/UVA-796

题目：裸的求桥，按第一个元素升序输出即可。

 #include <iostream>
 #include <cstdio>
 #include <algorithm>
 #include <vector>
 using namespace std;
 #define pb push_back
 #define fi first
 #define se second

 const int N = (int)1e3+;
 int n,tot,tim;
 int head[N],dfn[N],low[N];
 struct node{
     int to;
     int nxt;
 }e[N*N];
 vector<pair<int,int> > cut;

 void init(){
     for(int i = ; i <= n; ++i){
         head[i] = -;
         dfn[i] = ;
     }
     tim = tot = ;
 }

 inline void add(int u,int v){
     e[tot].to = v;
     e[tot].nxt = head[u];
     head[u] = tot++;
 }

 void tarjan(int now,int pre){
     dfn[now] = low[now] = ++tim;
     int to;
     for(int o = head[now]; ~o; o = e[o].nxt){
         to = e[o].to;
         if(to == pre) continue;//双向边的判断
         if(!dfn[to]){
             tarjan(to,now);
             low[now] = min(low[now],low[to]);
             //桥的条件
             if(dfn[now] < low[to]) cut.pb(make_pair(min(now,to),max(now,to)));
         }
         else if(low[now] > dfn[to]) low[now] = dfn[to];
     }
 }

 void _ans(){

     int ans = cut.size();
     sort(cut.begin(),cut.end());
     printf("%d critical links\n",ans);
     for(int i = ; i < ans; ++i) printf("%d - %d\n",cut[i].fi,cut[i].se);
     printf("\n");
     cut.clear();
 }

 int main(){

     int u,v,m;
     while(~scanf("%d",&n)){
         init();

         for(int i = ; i < n; ++i){
             scanf("%d (%d)",&u,&m);
             for(int j = ; j < m; ++j){
                 scanf("%d",&v);
                 add(u,v);
             }
         }

         for(int i = ; i < n; ++i) if(!dfn[i]) tarjan(i,i);
         _ans();
     }

     return ;
 }