kuangbin专题 专题九 连通图 Critical Links UVA

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+10;
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 = 0; i <= n; ++i){
        head[i] = -1;
        dfn[i] = 0;
    }
    tim = tot = 0;
}

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
",ans);
    for(int i = 0; i < ans; ++i) printf("%d - %d
",cut[i].fi,cut[i].se);
    printf("
");
    cut.clear();
}

int main(){

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

        for(int i = 0; i < n; ++i){
            scanf("%d (%d)",&u,&m);
            for(int j = 0; j < m; ++j){
                scanf("%d",&v);
                add(u,v);
            }
        }

        for(int i = 0; i < n; ++i) if(!dfn[i]) tarjan(i,i);
        _ans();
    }

    return 0;
}