Luogu P3478 [POI2008]STA-Station

換根 DP 模板題

邊目錄要開雙倍空間

不是 DAG 圖要連雙向邊！！！

#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define N 2000005
ll first[N], Next[N], to[N], size[N], f[N], tot, n;
void add(ll x, ll y)
{
	Next[++tot] = first[x];
	first[x] = tot;
	to[tot] = y;
	return;
}
void dfs_lenth(ll u, ll fa, ll dep)
{
	size[u] = 1;
	f[1] += dep;
	for(ll i = first[u]; i; i = Next[i])
	{
		ll v = to[i];
		if(v == fa)
		{
			continue;
		}
		dfs_lenth(v, u, dep + 1);
		size[u] += size[v];
	}
	return;
}
void dp(ll u, ll fa)
{
	for(ll i = first[u]; i; i = Next[i])
	{
		ll v = to[i];
		if(v == fa)
		{
			continue;
		}
		f[v] = f[u] + n - 2 * size[v];
		dp(v, u);
	}
	return;
}
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);
	cin >> n;
	ll u, v;
	for(ll i = 1; i < n; i++)
	{
		cin >> u >> v;
		add(u, v);
		add(v, u);
	}
	dfs_lenth(1, 0, 0);
	dp(1, 0);
	ll ans = 1;
	for(ll i = 2; i <= n; i++)
	{
		ans = (f[ans] < f[i] ? i : ans);
	}
	cout << ans;
	return 0;
}