class Solution {
    public TreeNode mirrorTree(TreeNode root) {
        if(root == null) return null;

        TreeNode left = mirrorTree(root.left);
        TreeNode right = mirrorTree(root.right);
        root.left = right;
        root.right = left;

        return root;
    }
}

class Solution {
    public TreeNode mirrorTree(TreeNode root) {
        if(root == null) return null;
        Deque<TreeNode> queue = new ArrayDeque<>();
        queue.offer(root);

        while(!queue.isEmpty()){
            TreeNode cur = queue.poll();
            TreeNode tmp = cur.left;
            cur.left = cur.right;
            cur.right = tmp;

            if(cur.left != null) queue.offer(cur.left);
            if(cur.right != null) queue.offer(cur.right);
        }

        return root;
    }
}