POJ 3160 Father Christmas flymouse

很简单的一道题，先tarjan缩点，然后SPFA求最长路。

#include<cstdio>
#include<string.h>
#include<math.h>
#include<queue>
#define N 30100
#define M 150100
using namespace std;
struct edge{
	int v,next;
}edge[M];
int n,m;
int num[N];
int cnt,head1[N],head2[N];
int scc,index,dfn[N],low[N],belong[N],sum[N];
int top,sstack[N];
bool instack[N],vis[N];
int d[N],dis[N],res;
void addedge(int u,int v,int *head)
{
    edge[cnt].v=v;
    edge[cnt].next=head[u];
    head[u]=cnt++;
}
int MAX(int a,int b){
	return a>b?a:b;
}
int MIN(int a,int b){
	return a>b?b:a;
}
void tarjan(int u)
{
    dfn[u]=low[u]=++index;
    sstack[++top]=u;
    instack[u]=true;
    for(int j=head1[u];j!=-1;j=edge[j].next)
    {
        int v=edge[j].v;
        if(dfn[v]==0)
        {
            tarjan(v);
            low[u]=MIN(low[v],low[u]);
        }
        else if(instack[v])
        {
            low[u]=MIN(low[u],dfn[v]);
        }
    }
    if(dfn[u]==low[u])
    {
        scc++;
        while(1)
        {
            int tmp=sstack[top--];
            instack[tmp]=false;
            belong[tmp]=scc;
            sum[scc]+=num[tmp];
            if(tmp==u) break;
        }
    }
}
void SPFA(int u){
	int i;
	queue<int>que;
	que.push(u);
	dis[u]=sum[u],vis[u]=1;
	res=max(res,dis[u]);
	while(!que.empty()){
		int tem=que.front();
		que.pop();
		for(i=head2[tem];i!=-1;i=edge[i].next){
			int v=edge[i].v;
			if(dis[v]<dis[tem]+sum[v]){
				dis[v]=dis[tem]+sum[v];
				res=max(res,dis[v]);
				if(!vis[v]){
					vis[v]=1;
					que.push(v);
				}
			}
		}
		vis[tem]=0;
	}
}
void make_graph(){
	int i,j;
	for(i=1;i<=n;i++){
		for(j=head1[i];j!=-1;j=edge[j].next){
			int v=edge[j].v;
			if(belong[i]!=belong[v]){
				addedge(belong[i],belong[v],head2);
				d[belong[v]]++;
			}
		}
	}
}
int main(){
	int i;
	while(scanf("%d %d",&n,&m)==2){
		memset(head1,-1,sizeof(head1));cnt=0;
		memset(head2,-1,sizeof(head2));
		memset(sum,0,sizeof(sum));
		memset(dfn,0,sizeof(dfn));
		memset(instack,0,sizeof(instack));top=0;
		for(i=1;i<=n;i++){
			scanf("%d",&num[i]);
			if(num[i]<=0)
				num[i]=0;
		}
		for(i=1;i<=m;i++){
			int u,v;
			scanf("%d %d",&u,&v);
			u++,v++;
			addedge(u,v,head1);
		}
		scc=0;
		for(i=1;i<=n;i++)
			if(!dfn[i])
				tarjan(i);                          //  强连通tarjan的模板
		memset(d,0,sizeof(d));
		make_graph();
		memset(dis,0,sizeof(dis));
		memset(vis,0,sizeof(vis));
		res=0;
		for(i=1;i<=scc;i++)
			if(!d[i]){
				memset(vis,0,sizeof(vis));
				SPFA(i);
			}
		printf("%d\n",res);
	}
}