HDU 3072 Intelligence System （强连通+（贪心||树形图））

大意：在一个联通分量里面的边权值忽略不计，求缩点后的所有联通分量链接在一起的最小权值和。

思路：必然先缩点，最后得到的图为DAG，然后就是统计权值，对于那么到达当前的缩点可能有多个入度，所以选择最小的即可。最后避开起始点的dis[star] == inf.累加即可。

#include<map>
#include<queue>
#include<cmath>
#include<cstdio>
#include<stack>
#include<iostream>
#include<cstring>
#include<algorithm>
#define LL int
#define inf 0x3f3f3f3f
#define eps 1e-8
#include<vector>
#define ls l,mid,rt<<1
#define rs mid+1,r,rt<<1|1
#define LL __int64
using namespace std;

const int nv = 50100;
const int ne = 101000;
struct node{
    int to,w,next;
}q[ne<<1];

const int Ma = nv;
int head[ne<<1],dfn[Ma],num[Ma],du[Ma],stk[Ma],vis[Ma],low[Ma];
int cnt,top,tim,scc,dis[Ma];
LL ans;

void Add(int a,int b,int c){
    q[cnt].to = b;
    q[cnt].w = c;
    q[cnt].next = head[a];
    head[a] = cnt++;
}

void init(){
    ans = scc =  cnt = top = 0;
    tim =  1;
    memset(head,-1,sizeof(head));
    memset(dfn,0,sizeof(dfn));
    memset(num,0,sizeof(num));
    memset(vis,0,sizeof(vis));
    memset(low,0,sizeof(low));
}

void Tarjan(int u){
    low[u] = dfn[u] = tim++;
    vis[u] = 1;
    stk[top++] = u;
    for(int i = head[u]; ~i ; i = q[i].next){
        int v = q[i].to;
        if(!dfn[v]){
            Tarjan(v);
            low[u] = min(low[u],low[v]);
        }
        else if(vis[v])
            low[u] = min(low[u],dfn[v]);
    }

    if(low[u] == dfn[u]){
        scc++;
        while(top > 0&&stk[top] != u){
            top --;
            vis[stk[top] ] = 0;
            num[stk[top] ] = scc;
        }
    }
}

int main(){
    int n,m,i,j,k,a,b,c,cla;
    while(~scanf("%d%d",&n,&m)){
        init();
        for(i = 0;i <m;++ i){
            scanf("%d%d%d",&a,&b,&c);
            Add(a,b,c);
        }

        for(i = 0;i < n;++ i)
            if(!dfn[i])
                Tarjan(i);

        memset(dis,inf,sizeof(dis));
        for(i = 0;i < n;++ i)
            for(j = head[i];~j;j=q[j].next){
                int v = q[j].to;
                if(num[i] != num[v]){
                    dis[ num[v] ] = min(dis[num[v] ],q[j].w);
                }
            }

        for(i = 1;i <= scc;++ i){
            if(dis[i]!=inf)
                ans+=dis[i];
        }
        printf("%I64d\n",ans);

    }
    return 0;
}