Aizu - 2784 Similarity of Subtrees（hash+dfs）

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<vector>
#include<map>

using namespace std;
typedef long long ll;
const int N = 1000005,p = 9901,MOD = 1e9+7;
vector<ll> G
;
ll Hash
;
map<ll,ll> m;
map<ll,ll>::iterator it;
void dfs(ll u)
{
Hash[u] = 1;
for(int i = 0;i < G[u].size();i++)
{
ll v = G[u][i];
dfs(v);
Hash[u] = (Hash[u]+Hash[v]*p)%MOD;
}
m[Hash[u]]++;
}
int main()
{
int n,u,v;
cin >> n;
for(int i = 1;i <= n-1;i++)
{
scanf("%d%d",&u,&v);
G[u].push_back(v);
}
dfs(1);
ll ans = 0;
for(it = m.begin();it != m.end();it++)
ans += it->second*(it->second-1)/2;
printf("%lld\n",ans);
return 0;
}