[Usaco2015 Jan]Grass Cownoisseur 图论 tarjan spfa

来源：互联网发布：电视软件论坛编辑：程序博客网时间：2024/06/05 16:11

先缩点，对于缩点后的DAG，正反跑spfa，枚举每条边进行翻转即可

#include<cstdio>#include<cstring>#include<iostream>#include<queue>using namespace std;struct pp{    struct edge{        int u,v,w,next;    }ed[200005];    int e,head[100005];    pp(){        e=1;        memset(head,0,sizeof head);    }    void add(int u,int v,int w){        ed[e].u=u; ed[e].v=v; ed[e].w=w;        ed[e].next=head[u]; head[u]=e++;    }}; pp orgp,newp,newf;int dfn[100005],low[100005],q[100005];int top=0,tot=0,id[100005],size[100005];bool bo[100005]; void tarjan(int x){    dfn[x]=low[x]=++top;    q[top]=x; bo[x]=1;    for(int i=orgp.head[x];i;i=orgp.ed[i].next){        int v=orgp.ed[i].v;        if(!dfn[v]){            tarjan(v);            low[x]=min(low[x],low[v]);        }        else if(bo[v])            low[x]=min(low[x],dfn[v]);    }    if(dfn[x]==low[x]){        int y; tot++;        do{            y=q[top--];            bo[y]=0;            id[y]=tot;            size[tot]++;        }while(y!=x);    }} int dis[100005][2]; void spfa(pp &ppp,int x,int t){    memset(bo,0,sizeof bo);    queue<int > q; q.push(x);     dis[x][t]=size[x]; int now,v,w;    while(!q.empty())    {        now=q.front(); q.pop(); bo[now]=0;        for(int i=ppp.head[now];i;i=ppp.ed[i].next)        {            v=ppp.ed[i].v; w=ppp.ed[i].w;            if(dis[v][t]<dis[now][t]+w){                dis[v][t]=dis[now][t]+w;                if(!bo[v]){                    bo[v]=1;                    q.push(v);                }            }        }    }} int n,m; int main(){    //freopen("cown.in","r",stdin);    //freopen("cown.out","w",stdout);    scanf("%d%d",&n,&m);    int u,v;    for(int i=1;i<=m;i++){        scanf("%d%d",&u,&v);        orgp.add(u,v,1);    }    for(int i=1;i<=n;i++)        if(!dfn[i])            tarjan(i);    for(int i=1;i<=m;i++){        if(id[orgp.ed[i].u]!=id[orgp.ed[i].v]){            newp.add(id[orgp.ed[i].u],id[orgp.ed[i].v],size[id[orgp.ed[i].v]]);            newf.add(id[orgp.ed[i].v],id[orgp.ed[i].u],size[id[orgp.ed[i].u]]);        }    }    int ans=-0x7fffffff;    memset(dis,-0x3f,sizeof dis);    spfa(newp,id[1],0);    spfa(newf,id[1],1);    for(int i=1;i<newp.e;i++)    {        u=newp.ed[i].u;        v=newp.ed[i].v;        ans=max(ans,dis[u][1]+dis[v][0]-size[id[1]]);    }    printf("%d\n",ans);}

打的好蠢啊QAQ

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