func_code_string: #include <bits/stdc++.h>   using namespace std;   #pragma GCC target( avx2 )   #pragma GCC optimization( O3 )   #pragma GCC optimization( unrolled-loops )   int n, m, b, w, color[200010], ok[200010];   vector<int> a[200010];   void dfs(int u, int c) {     color[u] = c;     ok[u] = 1;     b += (c == 0);     w += (c == 1);     for (int i = 0; i < a[u].size(); i++)       if (!ok[a[u][i]]) dfs(a[u][i], 1 - c);   }   void solve() {     cin >> n >> m;     for (int i = 1; i <= n; i++) {       a[i].clear();       color[i] = 0;       ok[i] = 0;     }     int u, v;     for (int i = 0; i < m; i++) {       cin >> u >> v;       a[u].push_back(v);       a[v].push_back(u);     }     b = 0;     w = 0;     dfs(1, 0);     cout << min(b, w) <<   n ;     if (b < w)       for (int i = 1; i <= n; i++) {         if (!color[i]) cout << i <<    ;       }     else       for (int i = 1; i <= n; i++) {         if (color[i]) cout << i <<    ;       }     cout <<   n ;   }   int main() {     int t;     cin >> t;     while (t--) solve();   }   