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=1;i<=n;i++)

    {

        if(indegree[i]==0)

        {

            q.push(i);

            dist[i]=value[i];

        }

    }

    while(!q.empty())

    {

        now=q.front();

        ans[num++]=now;

        q.pop();

        for(i=index[now];i!=-1;i=edge[i].next)

        {

            adj=edge[i].to;

            indegree[adj]--;

            if(!indegree[adj])

                q.push(adj);

        }

    }

    for(i=1;i<num;i++)

    {

        //cout<<ans[i]<<" ";

        for(j=index[ans[i]];j!=-1;j=edge[j].next)

        {

            dist[edge[j].to]=max(dist[edge[j].to],dist[ans[i]]+edge[j].val);

        }

        if(outdegree[ans[i]]==0)

        {

            if(dist[ans[i]]>Max)

                Max=dist[ans[i]];

        }

    }

    //cout<<endl;

}

void init()

{

    memset(indegree,0,sizeof(indegree));

    memset(outdegree,0,sizeof(outdegree));

    memset(index,-1,sizeof(index));

    memset(vi,0,sizeof(vi));

    memset(ans,0,sizeof(ans));

    for(int i=0;i<=100010;i++)

        dist[i]=-inf;

    Max=-inf;

    e=0;

    num=1;

}

int main()

{

    int m,i,j,a,b,val;

    while(scanf("%d%d",&n,&m)!=EOF)

    {

        init();

        for(i=1;i<=n;i++)

            scanf("%d",&value[i]);

        for(i=1;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 0;

}

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