countEmpty

public int countEmpty() {
    return treeCrawler(overallRoot);
}

int count = 1;
private int treeCrawler(IntTreeNode root) {
    if (root == null) {
        return count;
    } else {
       treeCrawler(root.right);
       count++;
       treeCrawler(root.left);
    }
    return count;
}