Submission #2375488

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Source Code Expand

#include<bits/stdc++.h>
using namespace std;

#define int long long

#define rep(i,n) for(int i=0;i<(n);i++)
#define pb push_back
#define all(v) (v).begin(),(v).end()
#define fi first
#define se second
typedef vector<int>vint;
typedef pair<int,int>pint;
typedef vector<pint>vpint;

template<typename A,typename B>inline void chmin(A &a,B b){if(a>b)a=b;}
template<typename A,typename B>inline void chmax(A &a,B b){if(a<b)a=b;}


const int mod=1000000007;
inline void add(int &a,int b){
    a+=b;
    if(a>=mod)a-=mod;
}
int mpow(int n,int m){
    int ret=1;
    while(m){
        if(m&1)ret=ret*n%mod;
        n=n*n%mod;
        m>>=1;
    }
    return ret;
}
const int FACT_SIZE=1111111;
int fact[FACT_SIZE];
int inv[FACT_SIZE];
struct fact_exec{
    fact_exec(){
        fact[0]=1;
        for(int i=1;i<FACT_SIZE;i++)fact[i]=fact[i-1]*i%mod;
        inv[FACT_SIZE-1]=mpow(fact[FACT_SIZE-1],mod-2);
        for(int i=FACT_SIZE-2;i>=0;i--)inv[i]=inv[i+1]*(i+1)%mod;
    }
}factexec;
int nCk(int n,int k){
    if(n<0|k<0||k>n)return 0;
    return fact[n]*inv[k]%mod*inv[n-k]%mod;
}
int nPk(int n,int k){
    if(n<0||k<0||k>n)return 0;
    return fact[n]*inv[n-k]%mod;
}


int N;
vint G[1111];

int dist[1111];

void dfs(int v,int p,int d){
    dist[v]=d;
    for(auto u:G[v]){
        if(u==p)continue;
        dfs(u,v,d+1);
    }
}


int B=1500;
int dp[1111][3333];
int D;
void dfs2(int v,int p,int d){
    for(int i=-(D/2-d);i<=D/2-d;i++)dp[v][i+B]=1;
    for(auto u:G[v]){
        if(u==p)continue;
        dfs2(u,v,d+1);
        for(int i=-(D/2-d);i<=D/2-d;i++)dp[v][i+B]=dp[v][i+B]*(dp[u][i+1+B]+dp[u][i-1+B])%mod;
    }
}

signed main(){


    cin>>N;
    rep(i,N-1){
        int a,b;
        cin>>a>>b;
        a--;b--;
        G[a].pb(b);G[b].pb(a);
    }

    dfs(0,-1,0);
    int v=0;
    rep(i,N)if(dist[i]>dist[v])v=i;
    dfs(v,-1,0);

    int u=0;
    rep(i,N)if(dist[i]>dist[u])u=i;

    vint latte(N);
    rep(i,N)latte[i]=dist[i];
    dfs(u,-1,0);
    rep(i,N)chmax(latte[i],dist[i]);

    vint lis;
    rep(i,N)if(latte[i]<=(dist[v]+1)/2)lis.pb(i);

    D=dist[v];

    if(lis.size()==1){
        dfs2(lis[0],-1,0);
        int ans=0;
        for(int i=0;i<3333;i++)add(ans,dp[lis[0]][i]);
        cout<<ans<<endl;
    }
    else{
    }
    return 0;
}

Submission Info

Judge Result