Symmetric Tree Check

boolean isSymmetric(Node root) {
  return root == null || isMirror(root.left, root.right);
}
boolean isMirror(Node a, Node b) {
  if (a == null && b == null) return true;
  if (a == null || b == null) return false;
  return a.val == b.val
      && isMirror(a.left, b.right)
      && isMirror(a.right, b.left);
}

Queue q = new LinkedList<>();
q.offer(root.left); q.offer(root.right);
while (!q.isEmpty()) {
  Node a = q.poll(), b = q.poll();
  if (a == null && b == null) continue;
  if (a == null || b == null || a.val != b.val) return false;
  q.offer(a.left); q.offer(b.right);
  q.offer(a.right); q.offer(b.left);
}
return true;