class Solution {
    public int maxDepth(TreeNode root) {
        if (root == null) return 0;
        return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
    }
}

class Solution {
    public int maxDepth(TreeNode root) {
        if (root == null) return 0;
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        int depth = 0;
        while (!queue.isEmpty()) {
            int size = queue.size();
            while (size != 0) {
                TreeNode p = queue.poll();
                if (p.left != null) queue.offer(p.left);
                if (p.right != null) queue.offer(p.right); 
                size -= 1;
            }
            depth += 1;
        }
        return depth;
    }
}