Dog Snacks

Posted 2021-10-05 thusloop

Dog Snacks
设f[i]为以i为根节点离i最近的叶子节点的距离
非根节点的子树间的转移为 k至少为f[i]+2
如果是根节点 则为最大子树k至少为f[i]+1;

#include<bits/stdc++.h>
#define int long long
#define ios ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
using namespace std;
const int inf=2e18+100;
const int maxn=2e5+100;
vector<int>g[maxn];
int f[maxn],d[maxn];
int ans;
void dfs(int x,int fa)
{
	vector<int>tmp;
	for(auto it:g[x])
	{
		if(it==fa)continue;
		dfs(it,x);
		f[x]=min(f[it]+1,f[x]);
		tmp.push_back(f[it]);
	}
	if(tmp.empty())return ;
	sort(tmp.begin(),tmp.end());
	if(tmp.size()>=2)ans=max(ans,tmp[tmp.size()-2]+2);
	if(x==1)ans=max(ans,tmp.back()+1);
	else ans=max(ans,tmp.back()+2);	
}
signed main()
{
	IOS
	int tt;
	cin>>tt;
	while(tt--)
	{
		ans=0;
		int n;
		cin>>n;
		for(int i=1;i<=n;i++)
		{
			f[i]=n+1;
			d[i]=0;
			g[i].clear();
		}
		for(int i=1;i<n;i++)
		{
			int u,v;
			cin>>u>>v;
			g[u].push_back(v);
			g[v].push_back(u);
			d[u]++;
			d[v]++;
		}
		for(int i=1;i<=n;i++)
		{
			if(d[i]==1)
			{
				f[i]=0;
			}
		}
		dfs(1,-1);
		cout<<ans<<"\\n";
	}
}