2016湖南省省赛 B 有向无环图（树形dp）

d[u]表示u到其他所有点的路径数*对应点的权值的和

#define bug(x) cout<<#x<<" is "<<x<<endl
#define IO std::ios::sync_with_stdio(0)
#include <bits/stdc++.h>
#define iter ::iterator
#define pa pair<int,ll>
#define pp pair<int,pa>
using namespace  std;
#define ll long long
#define mk make_pair
#define pb push_back
#define se second
#define fi first
#define ls o<<1
#define rs o<<1|1
const ll mod=1e9+7;
const int N=1e5+10;
int n,m;
vector<int>g[N];
ll a[N],b[N],vis[N];
ll d[N],in[N];
void dfs(int u){
    if(vis[u])return;
    vis[u]=1;
    d[u]=0;
    for(auto v:g[u]){
        dfs(v);
        d[u]=(d[u]+d[v]+b[v])%mod;
    }
}
int main(){
    while(~scanf("%d%d",&n,&m)){
        for(int i=1;i<=n;i++){
            g[i].clear();
            in[i]=0;
            d[i]=0;
        }
        for(int i=1;i<=n;i++){
            scanf("%lld%lld",&a[i],&b[i]);
        }
        for(int i=1;i<=m;i++){
            int x,y;
            scanf("%d%d",&x,&y);
            in[y]++;
            g[x].pb(y);
        }
        for(int i=1;i<=n;i++){
            if(!in[i])dfs(i);
        }
        ll ans=0;
        for(int i=1;i<=n;i++){
            ans=(ans+a[i]*d[i]%mod)%mod;
        }
        printf("%lld
",ans);
    }
}