POJ——T3160 Father Christmas flymouse

Time Limit: 1000MS		Memory Limit: 131072K
Total Submissions: 3496		Accepted: 1191

缩点，然后每个新点跑一边SPFA 

思路不难 ，注意细节~

#include <algorithm>
#include <cstring>
#include <cstdio>
#include <queue>

using namespace std;

const int M(150000+5);
const int N(30000+15);
int n,m,u,v,val[N],ans;

int hed[N],had[N],sumedge;
struct Edge
{
    int v,next;
}edge[M];
void ins(int u,int v,int *head)
{
    sumedge++;
    edge[sumedge].v=v;
    edge[sumedge].next=head[u];
    head[u]=sumedge;
}

int dfn[N],low[N],tim;
int col[N],sumcol,cval[N];
int Stack[N],instack[N],top;
void DFS(int now)
{
    dfn[now]=low[now]=++tim;
    Stack[++top]=now,instack[now]=1;
    for(int i=hed[now];i;i=edge[i].next)
    {
        int to=edge[i].v;
        if(!dfn[to]) DFS(to),low[now]=min(low[now],low[to]);
        else if(instack[to]) low[now]=min(low[now],dfn[to]);
    }
    if(low[now]==dfn[now])
    {
        col[now]=++sumcol;
        cval[sumcol]+=val[now];
        for(;Stack[top]!=now;top--)
        {
            col[Stack[top]]=sumcol;
            cval[sumcol]+=val[Stack[top]];
            instack[Stack[top]]=0;
        }
        instack[now]=0;top--;
    }
}

int rd[N];
void Get_map()
{
    for(u=1;u<=n;u++)
        for(int i=hed[u];i;i=edge[i].next)
        {
              v=edge[i].v;
              if(col[u]!=col[v])
              rd[col[v]]++,ins(col[u],col[v],had);
        }
}

queue<int>que;
int inq[N],dis[N];
int SPFA(int s)
{
    int ret=dis[s]=cval[s];
    memset(inq,0,sizeof(inq));
    que.push(s);inq[s]=1;
    while(!que.empty())
    {
        u=que.front();que.pop(),inq[u]=0;
        for(int i=had[u];i;i=edge[i].next)
        {
            v=edge[i].v;
            if(dis[v]<dis[u]+cval[v])
            {
                dis[v]=dis[u]+cval[v];
                ret=max(ret,dis[v]);
                if(!inq[v])
                    que.push(v),inq[v]=1;
            }
        }
    }
    return ret;
}

void init()
{
    sumcol=sumedge=top=tim=ans=0;
    memset(rd,0,sizeof(rd));
    memset(hed,0,sizeof(hed));
    memset(had,0,sizeof(had));
    memset(dis,0,sizeof(dis));
    memset(col,0,sizeof(col));
    memset(dfn,0,sizeof(dfn));
    memset(low,0,sizeof(low));
    memset(val,0,sizeof(val));
    memset(cval,0,sizeof(cval));
    memset(Stack,0,sizeof(Stack));
    memset(instack,0,sizeof(instack));
}

int main()
{
    while(~scanf("%d%d",&n,&m))
    {
        init();
        for(int i=1;i<=n;i++)
            scanf("%d",val+i),val[i]=max(0,val[i]);
        for(int u,v;m--;)
            scanf("%d%d",&u,&v),ins(u+1,v+1,hed);
        for(int i=1;i<=n;i++) if(!dfn[i]) DFS(i);
        Get_map();
        for(int i=1;i<=sumcol;i++)
            if(!rd[i]) ans=max(ans,SPFA(i));
        printf("%d
",ans);
    }
    return 0;
}