换根dp

#include <iostream>
#include <vector>
using namespace std;
using ll = long long ;
const int N=2e5+10;
ll f[N],g[N],size1[N];
vector<int>gg[N];
void dfs1(int x,int fa){
    size1[x]=1;
    for(auto &t:gg[x]){
        if(t==fa)continue;
        dfs1(t,x);
        f[x]+=f[t];
        size1[x]+=size1[t];
    }
    f[x]+=size1[x];
}
ll ans,n;
void dfs2(int x,int fa){
    if(x!=1){
        g[x]=g[fa]+n-2*size1[x];
        ans=max(ans,g[x]);
    }
    for(auto &t:gg[x]){
        if(fa==t)continue;
        dfs2(t,x);
    }
}
int main()
{
    ios::sync_with_stdio(false);
    cin >> n;
    for(int i=1,u,v;i<n;i++){
        cin>>u>>v;
        gg[u].push_back(v);
        gg[v].push_back(u);
    }
    dfs1(1,-1);
    ans=g[1]=f[1];
    dfs2(1,-1);
    cout<<ans;
    return 0;
}