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  1. #include<bits/stdc++.h>
  2. using namespace std;
  3.  
  4. #define ll long long
  5. #define all(v) v.begin(), v.end()
  6. #define memo(v, x) memset(v, x, sizeof(v))
  7. #define sz(v) (int)v.size()
  8.  
  9. void solve() {
  10.     int n, m; cin >> n >> m;
  11.     vector<vector<int>> adj(n);
  12.     for (int i = 0; i < m; i++) {
  13.         int u, v;
  14.         cin >> u >> v;
  15.         adj[u].push_back(v);
  16.         adj[v].push_back(u);
  17.     }
  18.     vector<int> comp, vis(n);
  19.     function<void(int)> dfs = [&](int node) {
  20.         comp.push_back(node);
  21.         vis[node] = 1;
  22.         for (auto &child : adj[node]) {
  23.             if (!vis[child]) {
  24.                 dfs(child);
  25.             }
  26.         }
  27.     };
  28.     vector<vector<int>> comps;
  29.     for (int i = 0; i < n; i++) {
  30.         if (!vis[i]) {
  31.             comp = vector<int>();
  32.             dfs(i);
  33.             comps.push_back(comp);
  34.         }
  35.     }
  36.     vector<array<int, 2>> ans;
  37.     priority_queue<array<int, 2>> pq;
  38.     for (int i = 0; i < sz(comps); i++) {
  39.         pq.push({sz(comps[i]), i});
  40.     }
  41.     while (sz(pq) > 1) {
  42.         auto [sza, ida] = pq.top();
  43.         pq.pop();
  44.         auto [szb, idb] = pq.top();
  45.         pq.pop();
  46.         ans.push_back({comps[ida].back(), comps[idb].back()});
  47.         comps[ida].pop_back(), comps[idb].pop_back();
  48.         sza--, szb--;
  49.         if (sza) pq.push({sza, ida});
  50.         if (szb) pq.push({szb, idb});
  51.     }
  52.     cout << sz(ans) << '\n';
  53.     for (auto &[u, v] : ans) {
  54.         cout << u << " " << v << '\n';
  55.     } 
  56. }
  57.  
  58. int32_t main() {
  59.     cin.tie(0) -> sync_with_stdio(0);
  60. #ifndef ONLINE_JUDGE
  61.     freopen("input.txt", "r", stdin);
  62.     freopen("output.txt", "w", stdout);
  63. #endif
  64.     int tc = 1;
  65.     // cin >> tc;
  66.     for (int t = 1; t <= tc; t++) {
  67.         solve();
  68.     }
  69.     return 0;
  70. }
Success #stdin #stdout 0s 5320KB
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https://ideone.com/w3sSw2
language:
C++ (gcc 8.3)
created:
3 hours ago
visibility:
public

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