CodeForces Round 718D - Andrew and Chemistry

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

#define LL long long
const int maxn = 112345;

vector<int> edge[maxn];
int dep[maxn],fa[maxn];

void getdep(int st,int Fa,int deep){
dep[st] = deep,fa[st] = Fa;
for(auto x : edge[st]){
if(x == Fa) continue;
getdep(x,st,deep+1);
}
}

pair<int,int> getCtr(int n){
int rot;
getdep(1,0,0);
rot = max_element(dep+1,dep+1+n) - dep;
getdep(rot,0,0);
rot = max_element(dep+1,dep+1+n) - dep;
int far = dep[rot];
for(int i = 0 ; i < far / 2 ; i ++)
rot = fa[rot];
return make_pair(rot,(far & 1) ? fa[rot] : rot);
}

const int mod = 1e9 + 7,seed = 17;
LL har[maxn];

LL getHar(int st,int fa){
har[st] = seed;
for(auto x : edge[st]){
if(x == fa) continue;
getHar(x,st);
}
sort(edge[st].begin(),edge[st].end(),[&](int x,int y){return har[x] < har[y];});
for(auto x : edge[st]){
if(x == fa) continue;
har[st] = (har[st] + har[x]) ^ (har[st] * har[x]);
har[st] %= mod;
}
(har[st] *= har[st]) %= mod;
return har[st];
}

int getans(int st,int fa){
int ret = edge[st].size() < 4;
int bef = -1;
for(auto x : edge[st]){
if(x == fa) continue;
if(bef == -1 || har[bef] != har[x])
ret += getans(x,st);
bef = x;
}
return ret;
}

int calans(int n){
pair<int,int> ctr = getCtr(n);
getHar(ctr.first,ctr.second);
getHar(ctr.second,ctr.first);
int ret = getans(ctr.first,ctr.second)+ getans(ctr.second,ctr.first);
return ret / (1+ (har[ctr.first] == har[ctr.second]));
}
int main(){
int n;
scanf("%d",&n);
int l,r;
for(int i = 1; i < n;i++){
scanf("%d %d",&l,&r);
edge[l].push_back(r);
edge[r].push_back(l);
}
printf("%d\n",calans(n));
}