static public String graphToString(TreeMap<Integer, TreeSet<Integer> > noeuds)
	 {
		 String strGraph = "";
		 for(Map.Entry<Integer,TreeSet<Integer>> entry : noeuds.entrySet())
		 {
			 Integer key = entry.getKey();
			 TreeSet<Integer> set = entry.getValue();
			 for(Integer value : set)
			 {
				 strGraph += key + " " + value + "\n";
			 }
		 }
		 return strGraph;
	 }