public void completeToLevel(int level) {
    // Added in exception check since we didn't get to that
    if (level < 1) {
        throw new IllegalArgumentException();
    }
    overallRoot = completeToLevel(overallRoot, 1, level);
}

private IntTreeNode completeToLevel(IntTreeNode root, int curr, int level) {
    if (curr <= level) {
        if (root == null) {
            root = new IntTreeNode(-1);
        }
        root.left = completeToLevel(root.left, curr + 1, level);
        root.right = completeToLevel(root.right, curr + 1, level);
    }
    return root;
}

/* This is the incorrect version I wrote during the A lecture.
 * If we had time in lecture to **Verify** our solution, we would have caught it and fixed it
 * like we should on the final exam. It's a good exercise to figure out WHY this doesn't work in
 * all cases. This is why verifying is important!

private IntTreeNode completeToLevel(IntTreeNode root, int curr, int level) {
    if (root == null) {
        if (curr <= level) {
            root = new IntTreeNode(-1);
        }
    } else {
        root.left = completeToLevel(root.left, curr + 1, level);
        root.right = completeToLevel(root.right, curr + 1, level);
    }
    return root;
}
 */