CF-1029-F. Wildflower · znzryb
## 思路讲解
注意到，根据题设条件，树必须为严格的“Y”||”1”。
## AC代码
::: 源代码
```cpp
// Problem: F. Wildflower F. 野花
// Contest: Codeforces - Codeforces Round 1029 (Div. 3)Codeforces 第 1029 轮（第 3 组）
// URL: https://codeforces.com/contest/2117/problem/F
// Memory Limit: 256 MB
// Time Limit: 2000 ms
//
// Powered by CP Editor (https://cpeditor.org)
#include <bits/stdc++.h>
#define all(vec) vec.begin(),vec.end()
#define CLR(i,a) memset(i,a,sizeof(i))
#define fi first
#define se second
#define pb push_back
#define SZ(a) ((int) a.size())
#define FOR(i, a, b) for (int i = (a); i <= (b); ++i)
#define ROF(i, a, b) for (int i = (a); i >= (b); --i)
#define debug(var) cerr << #var <<":"<<var<<"\n";
#define lson(var) (var<<1)
#define rson(var) ((var<<1)+1)
using namespace std;
typedef long long ll;typedef unsigned long long ull;
typedef double DB;typedef long double LD;
typedef __int128 i128;typedef pair<DB,DB> pdd;typedef pair<ll,bool> plb;
typedef pair<ll,ll> pll;
typedef array<ll,3> arr3;typedef array<ll,2> arr2;
constexpr ll MAXN=static_cast<ll>(1e6)+10,INF=static_cast<ll>(1e9)+9; // 1e18+9也是素数
constexpr ll mod=static_cast<ll>(1e9)+7;
constexpr double eps=1e-8;
ll N,M,Q,X,K,lT;
/*
*/
vector<ll> g[MAXN]; // 记得要清空
ll vis[MAXN],dp[MAXN];
inline void init(){
FOR(i,0,N+5){
g[i].clear();
}
FOR(i,0,N+5){
vis[i]=false;
}
}
ll tNode=0,ls=0,rs=0;
ll dfs(ll node){
ll ret=0;
vis[node]=true;
FOR(i,0,SZ(g[node])-1){
ll lnode=g[node][i];
if(vis[lnode]) continue;
ret+=dfs(lnode);
}
if(ret==0) ret=1;
return ret;
}
ll sdfs(ll node){
ll ret=1,up=0;
vis[node]=true;
ll cnt=0;
FOR(i,0,SZ(g[node])-1){
ll lnode=g[node][i];
if(vis[lnode]) continue;
up=ret;
ret+=sdfs(lnode);
++cnt;
}
if(cnt==2){
tNode=node;
ls=up-1;
rs=ret-up;
}
return ret;
}
ll binpow(ll a,ll k,const ll &p){ // 迭代快速幂
ll res=1;
while(k>0){
if((k&1)==1) res=a*res%p;
a=a*a%p;
k>>=1;
}
return res;
}
inline void solve(){
cin>>N;
init();
FOR(i,1,N-1){
ll u,v;
cin>>u>>v;
g[u].pb(v);g[v].pb(u);
}
ll ret=dfs(1);
FOR(i,0,N+5){
vis[i]=false;
}
if(ret==1){
cout<<binpow(2,N,mod)<<"\n";
}else if(ret==2){
sdfs(1);
ll line=N-ls-rs;
ll mx=max(ls,rs),mn=min(ls,rs);
ll ans=binpow(2,line,mod)*binpow(2,mx-mn,mod);
ans+=binpow(2,line,mod)*binpow(2,mx-mn-1,mod);
// if(mx-mn-1>=0){ans+=binpow(2,line,mod)*binpow(2,mx-mn-1,mod);}
ans%=mod;
cout<<ans<<"\n";
#ifdef LOCAL
debug(line);
debug(ls);
debug(rs);
#endif
}else{
cout<<0<<"\n";
}
}
int main()
{
ios::sync_with_stdio(false);
cin.tie(0);cout.tie(0);
cin>>lT;
while(lT--){
solve();
}
// solve();
return 0;
}
/*
AC
https://codeforces.com/contest/2117/submission/323644092
*/
```
:::
## 心路历程（WA，TLE，MLE……）
注意，写两种dfs记得要清空vis数组。