Codeforces 219D Choosing Capital for Treeland 2次DP

//选择一个根使得变换最少边的方向使得能够到达所有点
#include <map>
#include <set>
#include <list>
#include <cmath>
#include <ctime>
#include <deque>
#include <stack>
#include <queue>
#include <cctype>
#include <cstdio>
#include <string>
#include <vector>
#include <climits>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <algorithm>
#define LL long long
#define PI 3.1415926535897932626
using namespace std;
int gcd(int a, int b) {return a % b == 0 ? b : gcd(b, a % b);}
const int MAXN = 200010;
const int MAXM = MAXN * 10;
struct Edge {
    int u,v,w;
    int next;
}edge[MAXM];
int head[MAXN],tot;

void init() {
    tot = 0;
    memset(head,-1,sizeof(head));
}

void add_edge(int u,int v,int w) {
    edge[tot].u = u;
    edge[tot].v = v;
    edge[tot].w = w;
    edge[tot].next = head[u];
    head[u] = tot++;
}

int dp[MAXN],f[MAXN];
void dfs1(int u,int pre) {
    dp[u] = 0;
    for (int i = head[u] ; i != -1 ; i = edge[i].next) {
        int v = edge[i].v;
        if (v == pre) continue;
        dfs1(v,u);
        dp[u] += dp[v] + edge[i].w;
    }
}

void dfs2(int u,int pre) {
    for (int i = head[u] ; i != -1 ; i = edge[i].next) {
        int v = edge[i].v,w = edge[i].w;
        if (v == pre) continue;
        f[v] = f[u] - w + (1 - w);
        dfs2(v,u);
    }
}

int main() {
    int N;
    while (cin >> N) {
        init();
        for (int i = 0 ; i < N - 1 ; i++) {
            int u,v;
            scanf("%d%d",&u,&v);
            add_edge(u,v,0);
            add_edge(v,u,1);
        }
        dfs1(1,-1);
        f[1] = dp[1];
       // cout << dp[1] << endl;
        dfs2(1,-1);
        int ans = N;
        for (int i = 1 ; i <= N ; i++) ans = min(ans,f[i]);
        cout << ans << endl;
        for (int i = 1 ; i <= N ; i++) if (f[i] == ans) printf("%d ",i);
        puts("");
    }
    return 0;
}