树形DP Choosing Capital for Treeland

给你一棵有向树，需要选定一个点为capital，满足翻转边数最小

思路:先求出1为capital 的答案，然后向下更新孩子节点 

dp[i]=dp[i-1]+judge(i);

#include<iostream>  
#include<cstdio>  
#include<cstring>  
#include<algorithm>  
#include<cmath>  
#include<queue>  
#include<stack>  
#include<vector>  
#include<set>  
#include<map>  
  
#define L(x) (x<<1)  
#define R(x) (x<<1|1)  
#define MID(x,y) ((x+y)>>1)  
  
#define bug printf("hihi
")  
  
#define eps 1e-8  
typedef __int64 ll;  
  
using namespace std;  
#define INF 0x3f3f3f3f  
#define N 200005  
  
int dp[N];  
int n;  
  
struct stud{  
  int to,ne,len;  
}e[N*2];  
int head[N];  
int num;  
  
inline void add(int u,int v,int len)  
{  
    e[num].to=v;  
    e[num].len=len;  
    e[num].ne=head[u];  
    head[u]=num++;  
}  
  
void dfs(int u,int pre)  
{  
    dp[u]=0;  
    for(int i=head[u];i!=-1;i=e[i].ne)  
    {  
        int to=e[i].to;  
        if(to==pre) continue;  
        dfs(to,u);  
        dp[u]+=dp[to]+e[i].len;  
    }  
}  
  
void dfsup(int u,int pre)  
{  
    for(int i=head[u];i!=-1;i=e[i].ne)  
    {  
        int to=e[i].to;  
        if(to==pre) continue;  
        dp[to]=dp[u];  
        if(e[i].len) dp[to]--;  
        else dp[to]++;  
        dfsup(to,u);  
    }  
}  
  
int main()  
{  
    int i,j;  
    while(~scanf("%d",&n))  
    {  
        memset(head,-1,sizeof(head));  
        num=0;  
        int u,v;  
        i=n-1;  
        while(i--)  
        {  
            scanf("%d%d",&u,&v);  
            add(u,v,0);  
            add(v,u,1);  
        }  
        dfs(1,-1);  
        dfsup(1,-1);  
        int ans=INF;  
        for(i=1;i<=n;i++)  
            ans=min(ans,dp[i]);  
        printf("%d
",ans);  
        vector<int>temp;  
        temp.clear();  
        for(i=1;i<=n;i++)  
            if(dp[i]==ans) temp.push_back(i);  
        for(i=0;i<temp.size();i++)  
        {  
            if(i) printf(" ");  
            printf("%d",temp[i]);  
        }  
        printf("
");  
    }  
    return 0;  
}