poj 3249 拓扑排序 and 动态规划

思路：我们首先来一遍拓扑排序，将点按先后顺序排列于一维数组中，然后扫描一遍数组，将每个点的出边所连接的点进行更新，即可得到最优解。

#include<iostream>
#include<cstdio>
#include<cstring>
#include<queue>
#include<algorithm>
#define inf 2000000010
#define Maxn 100010
#define Maxm 1000010
using namespace std;
int dist[Maxn],vi[Maxn],index[Maxn],e,value[Maxn],indegree[Maxn],outdegree[Maxn],ans[Maxn],num,n;
struct Edge{
    int to,next,val;
}edge[Maxm];
void addedge(int from ,int to ,int val )
{
    edge[e].to=to;
    edge[e].val=val;
    edge[e].next=index[from];
    index[from]=e++;
}
int Max=-inf;
void Topsort()
{
    queue<int> q;
    int i,j,k,now,adj;
    for(i=;i<=n;i++)
    {
        if(indegree[i]==)
        {
            q.push(i);
            dist[i]=value[i];
        }
    }
    while(!q.empty())
    {
        now=q.front();
        ans[num++]=now;
        q.pop();
        for(i=index[now];i!=-;i=edge[i].next)
        {
            adj=edge[i].to;
            indegree[adj]--;
            if(!indegree[adj])
                q.push(adj);
        }
    }
    for(i=;i<num;i++)
    {
        //cout<<ans[i]<<" ";
        for(j=index[ans[i]];j!=-;j=edge[j].next)
        {
            dist[edge[j].to]=max(dist[edge[j].to],dist[ans[i]]+edge[j].val);
        }
        if(outdegree[ans[i]]==)
        {
            if(dist[ans[i]]>Max)
                Max=dist[ans[i]];
        }
    }
    //cout<<endl;
}
void init()
{
    memset(indegree,,sizeof(indegree));
    memset(outdegree,,sizeof(outdegree));
    memset(index,-,sizeof(index));
    memset(vi,,sizeof(vi));
    memset(ans,,sizeof(ans));
    for(int i=;i<=;i++)
        dist[i]=-inf;
    Max=-inf;
    e=;
    num=;
}
int main()
{
    int m,i,j,a,b,val;
    while(scanf("%d%d",&n,&m)!=EOF)
    {
        init();
        for(i=;i<=n;i++)
            scanf("%d",&value[i]);
        for(i=;i<=m;i++)
        {
            scanf("%d%d",&a,&b);
            addedge(a,b,value[b]);
            indegree[b]++,outdegree[a]++;
        }
        Topsort();
        //for(i=1;i<=n;i++)
            //cout<<dist[i]<<endl;
        printf("%d\n",Max);
    }
    return ;
}