private void travelTree(TreeNode node, List<Integer> result) {
    	
    	//record
    	result.add(node.val);
    	
    	TreeNode left  = node.left;
    	TreeNode right = node.right;
    	
    	//travel left
    	if(null != left) travelTree(left, result);
    	
    	//travel right
    	if(null != right) travelTree(right, result);
    }