RE求调

#include <bits/stdc++.h>
using namespace std;
#define int long long
const int N=10005;
vector<int> h[N];
int n,ans,d1[N],d2[N];
void dfs(int u,int fa)
{
	d1[u]=d2[u]=0;
	for(int v=0;v<h[u].size();v++)
	{
		if(v==fa) continue;
		dfs(v,u);
		int t=d1[v]-1;
		if(t>d1[u]) d2[u]=d1[u],d1[u]=t;
		else d2[u]=t;
	}
	ans=max(ans,d1[u]+d2[u]);
}
signed main()
{
	cin>>n;
	for(int i=1;i<n;i++)
	{
		int x,y;
		cin>>x>>y;
		h[x].push_back(y);
		h[y].push_back(x);
	}
	dfs(1,0);
	cout<<ans<<endl;
	return 0;
}