public TreeNode invertTree_iterative(TreeNode root) {
		if (root == null)
			return root;
		Stack<TreeNode> stack = new Stack<TreeNode>();
		stack.push(root);
		while (!stack.isEmpty()) {
			TreeNode node = stack.pop();
			TreeNode left = node.left;
			node.left = node.right;
			node.right = left;
			if (node.left != null) {
				stack.push(node.left);
			}
			if (node.right != null) {
				stack.push(node.right);
			}
		}
		return root;
	}