BZOJ 4562: [Haoi2016]食物链 拓扑排序

建反图，跑一个拓扑排序dp即可.

Code:

#include <bits/stdc++.h>
#define ll long long
#define N 100005
#define setIO(s) freopen(s".in","r",stdin)
using namespace std;
ll f[N];
queue<int>q;
int edges,n,m;
int in[N],out[N],hd[N],to[N<<1],nex[N<<1],mk[N];
void add(int u,int v)
{
    nex[++edges]=hd[u],hd[u]=edges,to[edges]=v;
}
int main()
{
    int i,j;
    // setIO("input");
    scanf("%d%d",&n,&m);
    for(i=1;i<=m;++i)
    {
        int a,b;
        scanf("%d%d",&a,&b),add(b,a),++in[a],++out[b];
    }
    for(i=1;i<=n;++i)
    {
        if(in[i]==0 && out[i]!=0)
        {
            q.push(i),f[i]=1,mk[i]=1;
        }
    }
    for(;!q.empty();)
    {
        int u=q.front();q.pop();
        for(int i=hd[u];i;i=nex[i])
        {
            int v=to[i];
            f[v]+=f[u];
            --in[v];
            if(in[v]==0) q.push(v);
        }
    }
    ll ans=0;
    for(i=1;i<=n;++i)
    {
        if(out[i]==0 && !mk[i])
        {
            ans+=f[i];
        }
    }
    printf("%lld\n",ans);
    return 0;
}