Submission #1135817

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Source Code Expand

//In the Name of God
//Ya Ali
 
#include<bits/stdc++.h>
 
#define int long long
 
#define pb push_back
 
#define err(A) cout<<#A<<" = "<<(A)<<endl
 
using namespace std;
 
const int M=1e9+7+0;
const int maxn=1024;
 
int n;
 
vector<int> g[maxn];
 
int lvl[maxn];
 
int d0[maxn];
int d1[maxn];
 
int D,D1,D2;
int far;
 
int r0,r1;
 
int dp[maxn][maxn/2];
int cp[maxn][maxn/2];
 
int ans;
 
void pfs(int v,int par=n)
{
  lvl[v]=lvl[par]+1;
  for(int u:g[v])
    if(u!=par)
      pfs(u,v);
  if(lvl[v]>lvl[far])
    far=v;
}
 
void dfs(int v,int par=n)
{
  lvl[v]=lvl[par]+1;
  for(int i=0;i<=D2;i++)
    {
      if(D1-lvl[v]>=i)
	dp[v][i]=1;
      if(D1-lvl[v]>=i-1)
	cp[v][i]=1;
    }
  for(int u:g[v])
    if(u!=par)
      {
	dfs(u,v);
	for(int i=0;i<=D2;i++)
	  {
	    (dp[v][i]*=dp[u][i]+(i?dp[u][i-1]:0))%=M;
	    (cp[v][i]*=cp[u][i]+(i?cp[u][i-1]:0))%=M;
	  }
      }
}
 
int32_t main()
{
  ios::sync_with_stdio(0);cin.tie(0);
 
  cin>>n;
  for(int v,u,i=0;i<n-1;i++)
    {
      cin>>v>>u;
      v--,u--;
      g[v].pb(u);
      g[u].pb(v);
    }
 
  lvl[n]=-1;
  pfs(far);
  pfs(far);
  for(int i=0;i<n;i++)
    d0[i]=lvl[i];
  pfs(far);
  for(int i=0;i<n;i++)
    d1[i]=lvl[i];
 
  D=lvl[far];
  D1=(D+0)/2;
  D2=(D+1)/2;
  
  if(D&1)
    {
      for(int i=0;i<n;i++)
	if(d0[i]+d1[i]==D)
	  if(d0[i]==d1[i]-1)
	    r0=i;
	  else if(d0[i]==d1[i]+1)
	    r1=i;
      lvl[r1]=-1;
      dfs(r0,r1);
      lvl[r0]=-1;
      dfs(r1,r0);
      for(int i=0;i<=D2;i++)
	{
	  ans+=dp[r0][i]*cp[r1][i]%M;
	  ans+=cp[r0][i+1]*dp[r1][i]%M;
	  ans-=dp[r0][i]*dp[r1][i]%M;
	  ans-=dp[r0][i+1]*dp[r1][i]%M;
	  ans+=dp[r1][i]*cp[r0][i]%M;
	  ans+=cp[r1][i+1]*dp[r0][i]%M;
	  ans-=dp[r1][i]*dp[r0][i]%M;
	  ans-=dp[r1][i+1]*dp[r0][i]%M;
	  ans%=M;
	}
    }
  else
    {
      for(int i=0;i<n;i++)
	if(d0[i]+d1[i]==D and d0[i]==d1[i])
	  r0=i;
      dfs(r0);
      for(int i=0;i<=D2;i++)
	ans+=dp[r0][i];
    }
 
  ans=(ans%M+M)%M;
  
  cout<<ans<<endl;
    
  return 0;
}

Submission Info

Judge Result