public List<Integer> preorderTraversal(TreeNode root) {
		List<Integer> res = new LinkedList<Integer>();
		if (root == null)
			return res;
		Stack<TreeNode> stack = new Stack<TreeNode>();
		stack.push(root);
		while (!stack.isEmpty()) {
			TreeNode cur = stack.pop();
			res.add(cur.val);
			if (cur.right != null)
				stack.push(cur.right);
			if (cur.left != null)
				stack.push(cur.left);
		}
		return res;
	}