Submission #1202937

#TimeUsernameProblemLanguageResultExecution timeMemory
1202937byunjaewooStar Trek (CEOI20_startrek)C++20
100 / 100
52 ms19016 KiB
#include <bits/stdc++.h> #define int long long using namespace std; const int N=100010, Mod=1e9+7; int n, d, dp[N], ep[N], cx[N], cy[N], cnt[N], w[N], c[N], c1, c2, ans0, ans1, rans; vector<int> adj[N]; void dfs1(int curr, int prev) { for(int next:adj[curr]) if(next!=prev) { dfs1(next, curr); cnt[curr]+=1-dp[next]; if(!dp[next]) dp[curr]=true; } } void dfs2(int curr, int prev) { for(int next:adj[curr]) if(next!=prev) { if(cnt[curr]-(1-dp[next])==0 && !ep[curr]) ep[next]=true; dfs2(next, curr); } w[curr]=dp[curr]|ep[curr]; } void dfs3(int curr, int prev) { cx[curr]=1-dp[curr]; for(int next:adj[curr]) if(next!=prev) dfs3(next, curr); if(dp[curr]) { for(int next:adj[curr]) if(next!=prev && !dp[next]) cx[curr]+=cx[next]*(cnt[curr]==1); } else { for(int next:adj[curr]) if(next!=prev) cx[curr]+=cx[next]; } } void dfs4(int curr, int prev) { int sum=0, sum2=0; for(int next:adj[curr]) { if(next!=prev && !dp[next] && cnt[curr]<=2) sum+=cx[next]; if(next!=prev) sum2+=cx[next]; } for(int next:adj[curr]) if(next!=prev) { int tmp=((cnt[curr]-(1-dp[next])+ep[curr])>=1); cy[next]=1-tmp; if(!tmp || cnt[curr]-(1-dp[next])==0 && ep[curr]) cy[next]+=cy[curr]; if(!tmp) cy[next]+=sum2-cx[next]; else if(cnt[curr]-(1-dp[next])==1 && !ep[curr]) cy[next]+=sum-cx[next]*(!dp[next]); dfs4(next, curr); } cy[curr]+=1-w[curr]; if(!w[curr]) c[curr]=cx[curr]+cy[curr]-1; else if(cnt[curr]+ep[curr]>=2) c[curr]=0; else if(dp[curr]) c[curr]=cx[curr]; else c[curr]=cy[curr]; } int Pow(int a, int b) { if(!b) return 1; int ret=Pow(a, b/2); ret=(ret*ret)%Mod; if(b&1) ret=(ret*a)%Mod; return ret; } signed main() { ios_base::sync_with_stdio(0); cin.tie(0); cin>>n>>d; for(int i=1; i<n; i++) { int u, v; cin>>u>>v; adj[u].push_back(v), adj[v].push_back(u); } dfs1(1, 0), dfs2(1, 0), dfs3(1, 0), dfs4(1, 0); for(int i=1; i<=n; i++) ans0+=w[i]; for(int i=1; i<=n; i++) c1+=w[i], c2=(c2+c[i]*(1-2*w[i])+Mod)%Mod; int tmp=((((n*n)%Mod)*c1)%Mod+(n*c2)%Mod)%Mod; tmp=(tmp*Pow((c2+n*n)%Mod, Mod-2))%Mod; ans1=(Pow(Mod-c2, d-1)*(ans0-tmp+Mod))%Mod; ans1=(ans1+tmp*Pow(n, 2*d-2))%Mod; rans=(c[1]*(Pow(n, 2*d-1)-ans1+Mod))%Mod; if(w[1]) rans=(Pow(n, 2*d)-rans+Mod)%Mod; cout<<rans; return 0; }
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