E. Cover it!

https://codeforc.es/contest/1176/problem/E

题意：选不超n/2个点使得每个没选的最少连一个选了的点；

 思路：隔层取，答案是最少点数的；

#include<algorithm>
#include<set>
#include<queue>
#include<cmath>
#include<cstring>
#include<iostream>
#include<set>
#include<vector>
#include<queue>
#include<cmath>
#include<cstdio>
#include<map>
#include<stack>
#include<string>
#include<bits/stdc++.h>
using namespace std;

#define sfi(i) scanf("%d",&i)
#define sfs(i) scanf("%s",(i))
#define pri(i) printf("%d\n",i)
#define sff(i) scanf("%lf",&i)
#define ll long long
#define ull unsigned long long
#define mem(x,y) memset(x,y,sizeof(x))
#define INF 0x3f3f3f3f
#define eps 1e-16
#define PI acos(-1)
#define lowbit(x) ((x)&(-x))
#define zero(x) (((x)>0?(x):-(x))<eps)
#define fl() printf("flag\n")
#define MOD(x) ((x%mod)+mod)%mod
#define endl '\n'
#define pb push_back
#define lson rt<<1,l,mid
#define rson rt<<1|1,mid+1,r
#define FAST_IO ios::sync_with_stdio(false);cin.tie(0);cout.tie(0)

const int maxn=1e6+9;
const int mod=1e9+7;

inline ll read()
{
    ll f=1,x=0;
    char ss=getchar();
    while(ss<'0'||ss>'9')
    {
        if(ss=='-')f=-1;ss=getchar();
    }
    while(ss>='0'&&ss<='9')
    {
        x=x*10+ss-'0';ss=getchar();
    }    return f*x;
}

vector<int>g[maxn];
vector<int>sum[3];
int n,m;
bool vis[maxn];
void dfs(int u,int id,int fa)
{
    if(vis[u]) return ;
    sum[id].pb(u);
    vis[u]=1;
    for(int i=0;i<g[u].size();i++)
    {
        int v=g[u][i];
        if(v==fa) continue;
        dfs(v,id^1,u);
    }
}
int main()
{
    //FAST_IO;
    //freopen("input.txt","r",stdin);

    int t;
    cin>>t;
    while(t--)
    {
        cin>>n>>m;
        for(int i=0;i<=n;i++) vis[i]=0,g[i].clear();
        sum[1].clear();
        sum[0].clear();
        while(m--)
        {
            int u,v;
            cin>>u>>v;
            g[u].pb(v);
            g[v].pb(u);
        }
        dfs(1,1,-1);
        if(sum[1].size()>sum[0].size())
        {
            cout<<sum[0].size()<<endl;
            for(int i=0;i<sum[0].size();i++)
            {
                cout<<sum[0][i]<<" ";
            }
            cout<<endl;
        }
        else
        {
            cout<<sum[1].size()<<endl;
            for(int i=0;i<sum[1].size();i++)
            {
                cout<<sum[1][i]<<" ";
            }
            cout<<endl;
        }
    }
}