Codeforces Round #135 (Div. 2) D 树形dp

//任选一个根节点，用dfs搜两遍，第一遍找以该节点为根节点

//每一个节点的子树中的相反的边

//第二遍dfs以同样的根节点搜索一遍，记录从根节点到该节点

//正向边和反向边dp[u] = dp[root] - sum_0 + sum_1 ；

#include<cstdio>

#include<cstring>

#include<iostream>

using namespace std ;

const int maxn = 200010 ;

const int inf  = 0x7fffffff ;

int dp[maxn] ;

int vis[maxn] ;

struct Edge

{

int v ;

int flag ;

int next ;

}edge[maxn*2];

int head[maxn] ; int nedge = 0 ;

int st = 1;

int mi  ;

void addedge(int u , int v)

{

edge[nedge].v = v;

edge[nedge].next = head[u];

edge[nedge].flag = 1 ;

head[u] = nedge++ ;

edge[nedge].v = u ;

edge[nedge].next = head[v] ;

edge[nedge].flag = 0;

head[v] = nedge++;

}

void dfs(int u)

{

vis[u] = 1;

for(int i = head[u];i != -1 ;i = edge[i].next)

{

int v = edge[i].v ;

if(vis[v]) continue ;

if(edge[i].flag == 0)

dp[u]++ ;

dfs(v) ;

dp[u] += dp[v];

}

}

void dfs_s(int u , int sum_1 , int sum_0)

{

vis[u] = 1;

dp[u] = dp[st] + sum_1 - sum_0;

mi = min(mi , dp[u]) ;

for(int i = head[u];i != -1 ;i = edge[i].next)

{

int v = edge[i].v;

if(vis[v])continue ;

if(edge[i].flag == 1)

dfs_s(v , sum_1+1 , sum_0) ;

else

dfs_s(v , sum_1 ,sum_0+1);

}

}

void init()

{

memset(head , -1 ,sizeof(head));

memset(dp , 0 , sizeof(dp)) ;

nedge = 0;

memset(vis , 0 , sizeof(vis)) ;

}

int main()

{

// freopen("input.txt","r",stdin);

// freopen("output.txt","w",stdout) ;

int n ;

while(~scanf("%d" ,&n))

{

int u , v;

init() ;

for(int i = 1;i < n;i++)

{

scanf("%d%d" ,&u , &v) ;

addedge(u ,v);

}

mi = inf;

dfs(st) ;

memset(vis , 0 , sizeof(vis)) ;

dfs_s(st , 0 , 0);

int flag = 0 ;

printf("%d\n" ,mi);

for(int i = 1;i <= n;i++)

if(dp[i] == mi)

{

if(flag)printf(" ");

flag = 1;

printf("%d" ,i) ;

}

puts("") ;

}

return  0;

}