CodeForces 219D Choosing Capital for Treeland (树形DP)

#include<bits/stdc++.h>
using namespace std;

#define debug puts("YES");
#define rep(x,y,z) for(int (x)=(y);(x)<(z);(x)++)
#define ll long long

#define lrt int l,int r,int rt
#define lson l,mid,rt<<1
#define rson mid+1,r,rt<<1|1
#define root l,r,rt
#define mst(a,b) memset((a),(b),sizeof(a))
#define pii pair<int,int>
#define fi first
#define se second
#define mk(x,y) make_pair(x,y)
const int mod=1e9+7;
const int maxn=2e5+10;
const int ub=1e6;
ll powmod(ll x,ll y){ll t; for(t=1;y;y>>=1,x=x*x%mod) if(y&1) t=t*x%mod; return t;}
ll gcd(ll x,ll y){
    if(y==0) return x;
    return gcd(y,x%y);
}
int n,x,y;
vector<pii> g[maxn];
int dp[maxn];
int dfs1(int u,int pre){
    for(int i=0;i<g[u].size();i++){
        int v=g[u][i].fi,t=g[u][i].se;
        if(v==pre) continue;
        dp[u]+=dfs1(v,u);
        if(t==-1) dp[u]++;
    }
    return dp[u];
}
void dfs2(int u,int pre){
    for(int i=0;i<g[u].size();i++){
        int v=g[u][i].fi,t=g[u][i].se;
        if(v==pre) continue;
        dp[v]+=dp[u]-dp[v]+t;
        dfs2(v,u);
    }
}
int main(){
    cin>>n;
    rep(i,1,n){
        int x,y;
        cin>>x>>y;
        g[x].push_back(mk(y,1));
        g[y].push_back(mk(x,-1));
    }
    dfs1(1,0),dfs2(1,0);
    int ans=1e9;rep(i,1,n+1) ans=min(ans,dp[i]);
    cout<<ans<<"\n";rep(i,1,n+1) if(dp[i]==ans) cout<<i<<" ";
    return 0;
}