给你一个无根树，问你以哪个节点为根节点的时候得到所有点的深度之和最大

《贴一张 知乎大佬的一个解释》

#include<bits/stdc++.h>
using namespace std;
const int N = 2e6+10;
using ll = long long;
ll dep[N],sz[N];
ll ans[N];
vector<int>g[N];
int n;

void dfs(int u,int father){
	sz[u] = 1;
	dep[u] = dep[father]+1;
	for(auto &t:g[u]){
		if(t==father)continue;
		dfs(t,u);
		sz[u]+=sz[t];
	}	
}


void down(int u,int father){
	
	for(auto &t:g[u]){
		if(t==father)continue;
		ans[t] = ans[u]-2*sz[t]+sz[1];
		down(t,u);
	}
}


int main()
{
	cin>>n;
	for(int i=1;i<n;i++){
		int a,b;cin>>a>>b;
		g[a].push_back(b);
		g[b].push_back(a);
	}
	
	dfs(1,0);
	for(int i=1;i<=n;i++)ans[1]+=dep[i];
	//for(int i=1;i<=n;i++)cout<<sz[i]<<"\n";
	down(1,-1);
	ll res = ans[1],p = 1;
	for(int i=1;i<=n;i++)if(ans[i]>res)res=ans[i],p=i;
	cout<<p;
}