leetcode101. symmetric tree

public boolean (TreeNode root) {
    if (root == null) return true;
    LinkedList<TreeNode> queue = new LinkedList<>();
    queue.offer(root);
    while (!queue.isEmpty()) {
        int size = queue.size();
        for (int i = 0; i < size; i++) {
            TreeNode p = queue.poll();
            if (p == null) continue;
            queue.offer(p.left);
            queue.offer(p.right);
        }
        LinkedList<TreeNode> tmp = new LinkedList<>(queue);
        while (!tmp.isEmpty()) {
            TreeNode first = tmp.pollFirst();
            TreeNode last = tmp.pollLast();
            if (first == null || last == null) {
                if (first != null) return false;
                if (last != null) return false;
                continue;
            }
            if (first.val != last.val) return false;
        }
    }
    return true;
}

public boolean isSymmetric02(TreeNode root) {
    if (root == null) return true;
    return helper(root.left, root.right);
}
public boolean helper(TreeNode left, TreeNode right) {
    if (left == null) return right == null;
    if (right == null) return false;
    return left.val == right.val && helper(left.left, right.right) && helper(left.right, right.left);
}