(连通图 模板题 无向图求桥)Critical Links -- UVA -- 796

链接：

https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=737

http://acm.hust.edu.cn/vjudge/contest/view.action?cid=82833#problem/C

说实话还不是太懂，自己再写点题理解理解！

代码：

#include<cstdio>
#include<cstdlib>
#include<cmath>
#include<iostream>
#include<algorithm>
#include<cstring>
#include<vector>
using namespace std;
#define N 1005

struct Edge{int v, next;}e[N*N];
struct Bridge{int u, v;}bri[N];

int bnt, n;
int Head[N], cnt;
int dfn[N], low[N], f[N], Index;

bool cmp(Bridge a, Bridge b)
{
    if(a.u != b.u)
        return a.u < b.u;
    return a.v < b.v;
}

void Init()
{
    cnt = bnt = Index = 0;
    memset(low, 0, sizeof(low));
    memset(dfn, 0, sizeof(dfn));
    memset(f, 0, sizeof(f));

    for(int i=0; i<=n; i++)
    {
        Head[i] = -1;
        dfn[i] = 0;
    }
}

void AddEdge(int u, int v)
{
    e[cnt].v = v;
    e[cnt].next = Head[u];
    Head[u] = cnt++;
}

void TarJan(int u, int fa)
{
    f[u] = fa;
    low[u] = dfn[u] = ++Index;

    for(int j=Head[u]; j!=-1; j=e[j].next)
    {
        int v = e[j].v;

        if(!dfn[v])
        {
            TarJan(v, u);
            low[u] = min(low[u], low[v]);
        }
        else if(v!=fa)
            low[u] = min(low[u], dfn[v]);
    }
}

int main()
{
    while(scanf("%d", &n)!=EOF)
    {
        int i, j, u, v, m;

        Init();

        for(i=0; i<n; i++)
        {
            scanf("%d (%d)", &u, &m);

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

        for(i=0; i<n; i++)
        {
            if(!dfn[i])
                TarJan(i, i);
        }

        for(i=0; i<n; i++)
        {
            int u = f[i];
            if(low[i] > dfn[u])
            {
                bri[bnt].u = min(u, i);
                bri[bnt++].v = max(u, i);
            }
        }

        sort(bri, bri+bnt, cmp);

        printf("%d critical links
", bnt);

        for(i=0; i<bnt; i++)
            printf("%d - %d
", bri[i].u, bri[i].v);
        printf("
");
    }
    return 0;
}