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   public int maxDepth(TreeNode root) {
        if (root == null) return 0; // Base case: Empty tree
        int leftDepth = maxDepth(root.left); // Depth of left subtree
        int rightDepth = maxDepth(root.right); // Depth of right subtree
        return 1 + Math.max(leftDepth, rightDepth); // Add 1 for the current node
    }