CSES – Building Roads

#include<bits/stdc++.h>
#define int long long
#define fastio ios_base::sync_with_stdio(0);cin.tie(0)
using namespace std;
//declare
const int maxn = 1e5+5;
int n,m;
bool vis[maxn];
vector<int> G[maxn];
//
void dfs(int cur){
    if(vis[cur]) return;
    vis[cur] = 1;
    for(int adj:G[cur]){
        dfs(adj);
    }
}
signed main(){
    fastio;
    cin>>n>>m;
    for(int i=0;i<m;i++){
        int a,b; cin>>a>>b;
        G[a].push_back(b);
        G[b].push_back(a);
    }
    vector<int> ans;
    for(int i=1;i<=n;i++){
        if(!vis[i]){
            dfs(i);
            ans.push_back(i);
        }
    }
    cout<<(int)ans.size()-1<<"\n";
    for(int i=1;i<ans.size();i++){
        cout<<ans[i-1]<<" "<<ans[i]<<"\n";
    }
    return 0;
}