深度优先搜索（DFS）

#include <bits/stdc++.h>

using namespace std;
int a[111][111];
vector<int>ans;
int book[111];
void dfs(int n,int x)
{
    if(book[x]==1)return;
    book[x]=1;
    ans.push_back(x);
    for(int i=0; i<n; i++)
    {
        if(a[x][i]==0) continue;
        dfs(n,i);
    }
}


int main()
{
    int t,n,m,x,y;
    cin>>t;
    while(t--)
    {
        cin>>n>>m;
        memset(a,0,sizeof(a));
        memset(book,0,sizeof(book));
        ans.clear();
        while(m--)
        {
            cin>>x>>y;
            a[x][y]=a[y][x]=1;
        }
        dfs(n,0);

        for(int i=0; i<n; i++)
        {
            if(i==n-1)
                printf("%d\n",ans[i]);
            else
                printf("%d ",ans[i]);
        }
    }
    return 0;
}