Building Roads.cpp

#include<bits/stdc++.h>
using namespace std;
#define INF 2047483647
#define INFL 9223372036854775807
#define ll long long
#define pii pair<ll,ll>
#define F first
#define S second
#define mp make_pair
#define pb push_back
#define ull unsigned long long
#define M 1000000007
#define FASTIO ios_base::sync_with_stdio(false);cin.tie(NULL); cout.tie(NULL);
#define take(x) scanf("%d",&x)
#define DE(x) printf("\ndebug %d\n",x);
#define vout(x) for(int i=0;i<x.size();i++) printf("%d ",x[i]);
#define pie acos(-1)
#define MOD 998244353

const int N = 1e5+5;
vector<int>adj[N];
int vis[N];
void dfs(int u){
    vis[u] = 1;
    for(auto v:adj[u]){
        if(vis[v] == 0)
            dfs(v);
    }
}

int main(){
    int n,m;
    cin>>n>>m;
    vector<int>res;

    for(int i=0;i<m;i++){
        int a,b;
        cin>>a>>b;
        a--;b--;
        adj[a].pb(b);
        adj[b].pb(a);
    }
    int forest = 0;
    for(int i=0;i<n;i++){
        if( vis[i] == 0 ){
            forest++;
            res.pb(i);
            dfs(i);
        }
    }
    cout<<res.size()-1<<endl;
    for(int i=1;i<res.size();i++){
        cout<<res[i-1]+1<<" "<<res[i]+1<<endl;
    }
}