HDU 4738 Caocao's Bridges（割边）

Posted 2020-07-19 icode-xiaohu

　　乍一看一个模板题，仔细一看还是模板题，但是三个坑。1，不是连通图，放0个。2 守卫为0，放1个。 3注意重边。

#include<iostream>
#include<cstdio>
#include<vector>
#include<queue>
#include<algorithm>
#include<stack>
#include<cstring>
using namespace std;
#define maxn 1005
int head[maxn],dfn[maxn],low[maxn],guard[maxn][maxn],tot,n;
int vis[maxn],all,index,bri[maxn*maxn];
struct Edge
{
    int to,nxt,g;
} edge[maxn*maxn*2];
queue<int> que;
void bfs(int u)
{
    vis[u] = 1;
    que.push(u);
    while(!que.empty())
    {
        int now = que.front();
        que.pop();
        for(int i = head[now]; i != -1; i = edge[i].nxt)
        {
            int v = edge[i].to;
            if(!vis[v]) {que.push(v);vis[v] = 1;}
        }
    }
}
bool judge()
{
    memset(vis,0,sizeof(vis));
    int sum = 0;
    while(!que.empty()) que.pop();
    for(int i = 1; i <= n; i++)
    {
        if(!vis[i])
        {
            sum++;
            bfs(i);
        }
    }
    if(sum==1) return true;
    return false;
}
void addedge(int u,int v,int w)
{
    edge[tot].to = v;
    edge[tot].g = w;
    edge[tot].nxt = head[u];
    head[u] = tot++;
}
void init()
{
    all = index = 0;
    memset(low,0,sizeof(low));
    memset(dfn,0,sizeof(dfn));
}
void tarjan(int u,int fa)
{
    low[u] = dfn[u] = ++index;
    for(int i = head[u];i != -1;i = edge[i].nxt){
        int v = edge[i].to;
        if(!dfn[v]){
            tarjan(v,u);
            low[u] = min(low[u],low[v]);
            if(low[v] > dfn[u] && guard[u][v]!=-2&&guard[v][u]!=-2){
                bri[all++] = edge[i].g;
            }
        }
        else if(v != fa) low[u] = min(low[u],dfn[v]);
    }
}
int main()
{
    int m,a,b,w;
    while(~scanf("%d%d",&n,&m))
    {
        if(n+m == 0) break;
        memset(head,-1,sizeof(head));
        tot = 0;
        memset(guard,-1,sizeof(guard));
        for(int i = 0; i < m; i++)
        {
            scanf("%d%d%d",&a,&b,&w);
            if(guard[a][b]==-1 && guard[b][a]==-1)
            {
                addedge(a,b,w);
                addedge(b,a,w);
                guard[a][b] = guard[b][a] = w;
            }
            else {
                guard[a][b] = guard[b][a] = -2;
            }
        }
        if(!judge())
        {
            puts("0");
            continue;
        }
        init();
        tarjan(1,-1);
        if(all == 0){
            puts("-1");
            continue;
        }
        sort(bri,bri+all);
        if(bri[0] == 0) puts("1");
        else printf("%d\n",bri[0]);
    }
}