poj 2378

先dfs球每个点的子树总结点数（包括他自己）。。然后就显然了。。

# include <iostream>
# include <cstdio>
# include <algorithm>
# include <cstring>
# include <vector>
using namespace std;
int d[20000],pa[20000];
typedef struct NODE{
    int k;struct NODE* next;
}Node;
Node edge[2*20000],*G[10001],*tail = edge;
void add(int a,int b)
{
    tail->k = b;tail->next = G[a];G[a] = tail++;
}
int dfs(int u,int fa)
{
    d[u] = 1;
    pa[u] = fa;
    Node* i;
    for (i=G[u];i;i=i->next)
        if (i->k!=fa)
            d[u] += dfs(i->k,u);
    return d[u];
}
int main()
{
    int n,a,b,i,cnt=0;
    Node* p;
    scanf("%d",&n);
    for (i=0;i<n-1;++i)
    {
        scanf("%d%d",&a,&b);
        add(a,b);
        add(b,a);    
    }
    n = dfs(1,0);
    for (i=1;i<=n;++i)
    {
        int flag = 1;
        for (p=G[i];p;p=p->next)
            if ((p->k)!=pa[i]&&d[p->k]>(n/2))
                flag = 0;
        if ((n-d[i])>(n/2)) flag = 0;
        if (flag)
        {
            printf("%d\n",i);
            cnt++;    
        }    
    }
    if (!cnt) printf("NONE\n");
    return 0;
}