换根的dp数组要开ll

#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N = 1e6 + 5, INF = 0x3f3f3f3f;
const LL mod = 1e9 + 7;
int n;
vector<int> e[N];
LL dp1[N], dp2[N], siz[N];
void dfs1(int u, int fa)
{
	for(auto v : e[u])
		if(v != fa)
		{
			dfs1(v, u);
			siz[u] += siz[v];
			dp1[u] += dp1[v];
		}
	dp1[u] += siz[u];
	siz[u] ++;
}
void dfs2(int u, int fa)
{
	for(auto v : e[u])
		if(v != fa)
		{
			dp2[v] = dp1[u] + dp2[u] - dp1[v] - siz[v] + siz[1] - siz[v];
			dfs2(v, u);
		}
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> n;
    for(int i = 1; i < n; i ++) 
    {
    	int u, v;
    	cin >> u >> v;
    	e[u].push_back(v);
    	e[v].push_back(u);
    }
    dfs1(1, 0);
    dfs2(1, 0);
    LL mx = 0, res;
    for(int i = 1; i <= n; i ++)
    	if(dp1[i] + dp2[i] > mx) res = i, mx = dp1[i] + dp2[i];
    cout << res << '\n'; 
    return 0;
}