Submission #1132887

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Source Code Expand

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<set>
#include<map>
#include<queue>
#include<cassert>
#define PB push_back
#define MP make_pair
#define sz(v) (in((v).size()))
#define forn(i,n) for(in i=0;i<(n);++i)
#define forv(i,v) forn(i,sz(v))
#define fors(i,s) for(auto i=(s).begin();i!=(s).end();++i)
#define all(v) (v).begin(),(v).end()
using namespace std;
typedef long long in;
typedef vector<in> VI;
typedef vector<VI> VVI;
const in mdl=1000000007;
VVI egs;
in n;
VVI wy;
in wys(in u, in pr, in dnlim, in rad){
  in& tp=wy[u][dnlim];
  if(tp!=-1)
    return tp;
  tp=1;
  in tw;
  forv(i,egs[u]){
    if(egs[u][i]==pr)
      continue;
    tw=0;
    if(dnlim)
      tw+=wys(egs[u][i],u,dnlim-1,rad-1);
    if(dnlim!=rad)
      tw+=wys(egs[u][i],u,dnlim,rad-1);
    tp=tp*tw%mdl;
  }
  return tp;
}
VI dglft;
VI act;
VI trn;
int main(){
  ios::sync_with_stdio(0);
  cin.tie(0);
  cin>>n;
  egs.resize(n);
  dglft.resize(n);
  act.resize(n,1);
  trn.resize(n);
  in ta,tb;
  forn(z,n-1){
    cin>>ta>>tb;
    --ta;
    --tb;
    egs[ta].PB(tb);
    egs[tb].PB(ta);
    ++dglft[ta];
    ++dglft[tb];
  }
  VI trm;
  in ct=0;
  forn(i,n){
    if(dglft[i]==1){
      trn[i]=ct;
      trm.PB(i);
      act[i]=0;
    }
  }
  wy.resize(n,VI(n+2,-1));
  in cd;
  while(1){
    ++ct;
    VI ttrm;
    forv(i,trm){
      forv(j,egs[trm[i]]){
	cd=egs[trm[i]][j];
	if(act[cd]){
	  --dglft[cd];
	  if(dglft[cd]<=1){
	    act[cd]=0;
	    trn[cd]=ct;
	    ttrm.PB(cd);
	  }
	}
      }
    }
    trm=ttrm;
    if(trm.empty())
      break;
  }
  in mxd=-1;
  VI cent;
  forn(i,n){
    if(trn[i]>mxd){
      mxd=trn[i];
      cent.clear();
    }
    if(trn[i]==mxd)
      cent.PB(i);
  }
  in rad=mxd;
  bool centeg=(sz(cent)==2);
  assert(sz(cent)==1 || sz(cent)==2);
  in sm=0;
  if(centeg){
    forn(xx,2){
      forn(zz,2){
	forn(dnlim,rad+1){
	  in ls=wys(cent[0],cent[1],dnlim,rad);
	  in rs=wys(cent[1],cent[0],dnlim+zz,rad+1);
	  sm+=ls*rs;
	  sm%=mdl;
	}
      }
      forn(i,n){
	forv(j,wy[i])
	  wy[i][j]=-1;
      }
      swap(cent[0],cent[1]);
    }
    forn(xx,2){
      forn(dnlim,rad+1){
	for(in z=0;z<2 && dnlim+z<=rad;++z){
	  in ls=wys(cent[0],cent[1],dnlim,rad);
	  in rs=wys(cent[1],cent[0],dnlim+z,rad);
	  sm-=ls*rs;
	  sm%=mdl;
	}
      }
      forn(i,n){
	forv(j,wy[i])
	  wy[i][j]=-1;
      }
      swap(cent[0],cent[1]);
    }
    sm%=mdl;
    sm+=mdl;
    sm%=mdl;
  }
  else{
    forn(dnlim,rad+1){
      sm+=wys(cent[0],cent[0],dnlim,rad);
      sm%=mdl;
    }
  }
  cout<<sm<<endl;
  return 0;
}

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