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(+);
 const int N(+);
 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]=;
     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]]=;
         }
         instack[now]=;top--;
     }
 }

 int rd[N];
 void Get_map()
 {
     for(u=;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,,sizeof(inq));
     que.push(s);inq[s]=;
     while(!que.empty())
     {
         u=que.front();que.pop(),inq[u]=;
         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]=;
             }
         }
     }
     return ret;
 }

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

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