public ArrayList<Integer> postorderTraversal_iteration(TreeNode root) {
		ArrayList<Integer> nodes = new ArrayList<Integer>();
		if (root == null)
			return nodes;
		Stack<TreeNode> stack = new Stack<TreeNode>();
		TreeNode node;
		stack.push(root);
		while (!stack.isEmpty()) {
			node = stack.pop();
			nodes.add(node.val);
			if (node.left != null)
				stack.push(node.left);
			if (node.right != null)
				stack.push(node.right);

		}
		ArrayList<Integer> result = new ArrayList<Integer>();
		for (int i = nodes.size() - 1; i >= 0; i--) {
			result.add(nodes.get(i));
		}
		return result;
	}