Codeforces Round #192 (Div. 2) B. Road Construction

 #include <iostream>
 #include <vector>

 using namespace std;

 int main(){
     int n,m;
     cin >> n >> m;
     vector< bool > flag(n+,false);
     for(int i =  ; i < m ; i ++ ){
         int a,b;
         cin >> a>>b;
         flag[a]=flag[b]=true;
     }
     int centerPoint = ;
     for(int i =  ; i <= n ; i  ++ ){
         if(!flag[i]){
             centerPoint = i;
             break;
         }
     }
     cout<<n-<<endl;
     for(int i = ; i <= n ; i ++ ){
         if(i!=centerPoint) cout<<i<<" "<<centerPoint<<endl;
     }
     return ;
 }

主要思路：就是一个中心点，其余的点都挂在上面