codeforces 192e

link: http://codeforces.com/contest/330/problem/E

 /*
 ID: zypz4571
 LANG: C++
 TASK: 192e.cpp
  */

 #include <iostream>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
 #include <cmath>
 #include <cctype>
 #include <algorithm>
 #include <queue>
 #include <deque>
 #include <queue>
 #include <list>
 #include <map>
 #include <set>
 #include <vector>
 #include <utility>
 #include <functional>
 #include <fstream>
 #include <iomanip>
 #include <sstream>
 #include <numeric>
 #include <cassert>
 #include <ctime>
 #include <iterator>
 const int INF = 0x3f3f3f3f;
 const int dir[][] = {{-,},{,},{,-},{,},{-,-},{-,},{,-},{,}};
 using namespace std;
 map<pair<int,int>,bool> coll;
 int a[];
 int main ( int argc, char *argv[] )
 {
 #ifndef ONLINE_JUDGE
     //freopen("in.txt", "r", stdin);
 #endif
     int n, m; cin>>n>>m;
     double d=clock(); srand(time());
     for (int i = , x, y; i < m; ++i) {
         cin>>x>>y;
         coll.insert(make_pair(make_pair(x,y),true));
         coll.insert(make_pair(make_pair(y,x),true));
     }
     for (int i = ; i < n; ++i) a[i] = i+;
     bool t;
     while ((clock()-d)/CLOCKS_PER_SEC < 2.9) {
         random_shuffle(a, a+n);
         a[n] = a[]; t = false;
         for (int i = ; i < m; ++i)
             if (coll[make_pair(a[i],a[i+])]) {
                 t = true; break;
             }
         if (!t) break;
     }
     if (t) printf("-1\n");
     else for (int i = ; i < m; ++i) printf("%d %d\n", a[i], a[i+]);
         return EXIT_SUCCESS;
 }                /* ----------  end of function main  ---------- */

note the use of random_shuffle.

the possibility of each try of success is about (1-2/n)^n which is big enough when n is very large so we can use this nondeterminisitic solution.