public Node next(){
				Node rel=null;
				while(c!=null||!s.isEmpty()) {
					if (c!=null) {
						s.push(c);
						c=c.left;
					}
					if (c==null) {
						c=s.pop();
						rel=c;
						c=c.right;
						break;
					}
				}
				return rel;
			}