大佬求助

#include<bits/stdc++.h>
using namespace std;
#define int long long
int dp[5000005], sum[5000005], size[5000005], head[5000005], cnt;
struct node {
	int to, nxt;
}a[5000005];
int ans, maxx = -1e9, n;
void add(int u, int v) {
	a[++ cnt].to = v;
	a[cnt].nxt = head[u];
	head[u] = cnt;
}
void dfs1(int x, int fa) {
	size[x] = 1;
	sum[x] = sum[fa] + 1;
	for(int i = head[x];i != 0;i = a[i].nxt) {
		if(a[i].to != fa) {
			dfs1(a[i].to, x);
			size[x] += size[a[i].to];
			sum[x] += sum[a[i].to];
		}
	}
}
void dfs2(int x, int fa) {
	for(int i = head[x];i != 0;i = a[i].nxt) {
		if(a[i].to != fa){
			dp[a[i].to] = dp[x] - size[a[i].to] + (n - size[a[i].to]);
			dfs2(a[i].to, x);
		}
	}
}
signed main() {
	cin >> n;
	for(int i = 1;i < n;i ++) {
		int u, v;
		cin >> u >> v;
		add(u, v), add(v, u);
	}
	dfs1(1, 0);
	dp[1] = sum[1];
	dfs2(1, 0);
	for(int i = 1;i <= n;i ++) {
		if(maxx < dp[i]) {
			ans = i;
			maxx = dp[i];
		}
	}
	cout << ans;
}