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;
}