C++ 解法, 执行用时: 308ms, 内存消耗: 46980K, 提交时间: 2022-07-16 11:52:28

#include<bits/stdc++.h>
#define For(i,a,b) for(int i=(a),i##END=(b);i<=i##END;i++)
#define Rof(i,b,a) for(int i=(b),i##END=(a);i>=i##END;i--)
#define go(u) for(int i=head[u];i;i=nxt[i])
#define int long long
using namespace std;
inline int read(){
    int x=0,f=1;
    char ch=getchar();
    while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
    while(ch>='0'&&ch<='9'){x=(x<<1)+(x<<3)+(ch^48);ch=getchar();}
    return x*f;
}
const int N=2e3+10,p=1e9+7;
vector<int> g[N];
inline int inv(int a,int b=p-2){
	int x=1;while(b){
		if(b&1)(x*=a)%=p;
		(a*=a)%=p,b>>=1;
	}return x;
}
int C[N][N],iC[N][N];
void init(int n=N-10){
	For(i,0,n){C[i][0]=1;For(j,1,i)C[i][j]=(C[i-1][j-1]+C[i-1][j])%p;}
	For(i,0,n)For(j,0,i)iC[i][j]=inv(C[i][j]);
}
int dp[N],n,rt,sz[N],fa[N],ivdp[N];
void dfs1(int u,int f){
	sz[u]=dp[u]=1,fa[u]=f;
	for(int v:g[u])if(v!=f){
		dfs1(v,u),sz[u]+=sz[v];
		(dp[u]*=dp[v]*C[sz[u]-1][sz[v]]%p)%=p;
	}ivdp[u]=inv(dp[u]);
}int f[N][N],ans[N];
int get(int x,int y,int u){
	if(f[x][y])return f[x][y];
	int& w=f[x][y];
	if(fa[x]==u&&fa[y]==u)w=dp[u]*ivdp[x]%p*ivdp[y]%p*iC[sz[u]-1][sz[x]]%p*iC[sz[u]-1-sz[x]][sz[y]]%p*C[sz[u]-2][sz[x]+sz[y]-1]%p;
	if(fa[x]!=u)(w+=dp[fa[x]]*ivdp[x]%p*iC[sz[fa[x]]-1][sz[x]]%p*C[sz[fa[x]]+sz[y]-2][sz[fa[x]]-sz[x]-1]%p*get(fa[x],y,u)%p)%=p;
	if(fa[y]!=u)(w+=dp[fa[y]]*ivdp[y]%p*iC[sz[fa[y]]-1][sz[y]]%p*C[sz[fa[y]]+sz[x]-2][sz[fa[y]]-sz[y]-1]%p*get(x,fa[y],u)%p)%=p;
	return w;
}
vector<int> solve(int u,int f){
	vector<int> t,d;int iv=inv(sz[u]-1);
	for(int v:g[u])if(v!=f){
		(ans[u]+=dp[u]*iv%p*sz[v]%p*abs(u-v))%=p,d=solve(v,u);
		for(int a:t)for(int b:d)(ans[u]+=2*abs(a-b)%p*dp[a]%p*dp[b]%p*C[sz[a]+sz[b]-2][sz[a]-1]%p*get(a,b,u))%=p;
		for(int b:d)t.push_back(b);
	}t.push_back(u);return t;
}
void dfs2(int u,int f){
	int iv=inv(sz[u]-1);for(int v:g[u])if(v!=f)
		dfs2(v,u),(ans[u]+=dp[u]*ivdp[v]%p*iC[sz[u]-1][sz[v]]%p*C[sz[u]-2][sz[v]-1]%p*ans[v])%=p;
}
signed main(){
	init(),n=read(),rt=read();For(i,2,n){
		int u=read(),v=read();
		g[u].push_back(v),g[v].push_back(u);
	}dfs1(rt,0),solve(rt,0),dfs2(rt,0),cout<<ans[rt]<<endl;
	return 0;
}