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
1 #define bug(x) cout<<#x<<" is "<<x<<endl 2 #define IO std::ios::sync_with_stdio(0) 3 #include <bits/stdc++.h> 4 #define iter ::iterator 5 #define pa pair<int,ll> 6 #define pp pair<int,pa> 7 using namespace std; 8 #define ll long long 9 #define mk make_pair 10 #define pb push_back 11 #define se second 12 #define fi first 13 #define ls o<<1 14 #define rs o<<1|1 15 const ll mod=1e9+7; 16 const int N=1e5+10; 17 int n,m; 18 vector<int>g[N]; 19 ll a[N],b[N],vis[N]; 20 ll d[N],in[N]; 21 void dfs(int u){ 22 if(vis[u])return; 23 vis[u]=1; 24 d[u]=0; 25 for(auto v:g[u]){ 26 dfs(v); 27 d[u]=(d[u]+d[v]+b[v])%mod; 28 } 29 } 30 int main(){ 31 while(~scanf("%d%d",&n,&m)){ 32 for(int i=1;i<=n;i++){ 33 g[i].clear(); 34 in[i]=0; 35 d[i]=0; 36 } 37 for(int i=1;i<=n;i++){ 38 scanf("%lld%lld",&a[i],&b[i]); 39 } 40 for(int i=1;i<=m;i++){ 41 int x,y; 42 scanf("%d%d",&x,&y); 43 in[y]++; 44 g[x].pb(y); 45 } 46 for(int i=1;i<=n;i++){ 47 if(!in[i])dfs(i); 48 } 49 ll ans=0; 50 for(int i=1;i<=n;i++){ 51 ans=(ans+a[i]*d[i]%mod)%mod; 52 } 53 printf("%lld\n",ans); 54 } 55 }
