2020牛客暑期多校训练营（第二场）Cover the Tree

题目描述

Given an unrooted tree, you should choose the minimum number of chains that all edges in the tree are covered by at least one chain. Print the minimum number and one solution. If there are multiple solutions, print any of them.

输入描述:

输出描述:

连线的话最好是两个叶子节点一连，最后一个点随便连。
于是就要把叶子节点按dfs序标号，然后两个一输出

#include
#define ll long long
using namespace std;
ll a[200010],b[200020],n,m,i,j,k,l,o,p;
vector<ll> v[200100];
void dfs(ll pos,ll fa)//dfs标号
{
    if (a[pos]==1)
    {
        b[++m]=pos;
        return;
     }
    for (ll i=0;i<v[pos].size();i++)
     {
        if (v[pos][i]==fa) continue;
        dfs(v[pos][i],pos);
     }
}
int main()
{
    scanf("%lld",&n);
    if (n==2)//特判，如果就两个节点直接输出1
    {
        printf("1\n");
        return 0;
    }
    for (i=1;i<n;i++)
     {
        ll x,y;
        scanf("%lld%lld",&x,&y);
        a[x]++;a[y]++;
        v[x].push_back(y);
        v[y].push_back(x);
     }
    m=0;
    ll root=0;
    for (i=1;i<=n;i++)
     if (a[i]>1)
      {
        dfs(i,-1);
        root=i;
        break;
      }
    printf("%lld\n",(m+1)/2);
    for (i=1;i<=m/2;i++)
     printf("%lld %lld\n",b[i],b[m/2+i]);
    if (m%2) printf("%lld %lld\n",b[m/2+1],root);
}

然后只有80分。。。
查了之后
发现会被这个图卡住

继菊花图后的另一个奇怪的图
然后改一下输出顺序，AC

#include
#define ll long long
using namespace std;
ll a[200010],b[200020],n,m,i,j,k,l,o,p;
vector<ll> v[200100];
void dfs(ll pos,ll fa)
{
	if (a[pos]==1)
	{
		b[++m]=pos;
		return;
	 }
	for (ll i=0;i<v[pos].size();i++)
	 {
	 	if (v[pos][i]==fa) continue;
	  	dfs(v[pos][i],pos);
	 }
}
int main()
{
	scanf("%lld",&n);
	if (n==2)
	{
		printf("1\n");
		return 0;
	}
	for (i=1;i<n;i++)
	 {
	 	ll x,y;
	 	scanf("%lld%lld",&x,&y);
	 	a[x]++;a[y]++;
	 	v[x].push_back(y);
	 	v[y].push_back(x);
	 }
	m=0;
	ll root=0;
	for (i=1;i<=n;i++)
	 if (a[i]>1)
	  {
	  	dfs(i,-1);
	  	root=i;
	  	break;
	  }
	printf("%lld\n",(m+1)/2);
	for (i=1;i<=m/2;i++)
	 printf("%lld %lld\n",b[i],b[(m+1)/2+i]);
	if (m%2) printf("%lld %lld\n",b[m/2+1],root);
}