HDU - 6295 (violence)

Meaning of the questions: to give you a tree n dots, each dot random [1, n] weights, ask you how many trees there is a palindrome string.

Thinking: random weights are [1, n]. Consider the probability of a bold guess palindrome string length must be very short. The length of the violence at less than equal to the number 3 on the list.

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
vector<int> q[100005];
int a[100005];
int vis[100005];ll ans=0;
void dfs(int u,int fa)
{
	vis[a[u]]++;
	for(int i=0;i<q[u].size();i++)
	{
		int v=q[u][i];
		if(v==fa) continue;
		ans+=vis[a[v]];
		vis[a[v]]++;
		if(fa!=-1&&a[v]==a[fa]) ans++;
	}
	for(int i=0;i<q[u].size();i++)
	{
		int v=q[u][i];
		if(v==fa) continue;
		vis[a[v]]--;
	}
	vis[a[u]]--;
	for(int i=0;i<q[u].size();i++)
	{
		int v=q[u][i];
		if(v==fa) continue;
		dfs(v,u);
	}
}
int main()
{
	int t;scanf("%d",&t);
	while(t--)
	{
		int n;
		scanf("%d",&n);
		for(int i=1;i<=n;i++) q[i].clear();
		for(int i=1;i<=n;i++) scanf("%d",&a[i]);
		memset(vis,0,sizeof(vis));
		ans=0;
		for(int i=0;i<n-1;i++)
		{
			int x,y;
			scanf("%d%d",&x,&y);q[x].push_back(y);q[y].push_back(x);
		}
		dfs(1,-1);
		printf("%d\n",ans+n);
	}
	return 0;
}