public void firstTraverse(TreeNode tr,String path){
		if(tr.left==null && tr.right == null){
			path+=tr.val;
			res.add(path);
			return;
		}else{
			path= path+tr.val+"->";
		}
		if(tr.left!=null)
			firstTraverse(tr.left,path);
		if(tr.right!=null)
			firstTraverse(tr.right,path);
	}