Convert Sorted Array to Binary Search Tree

public TreeNode sortedArrayToBST(int[] num) {
    if (num.length == 0) {
        return null;
    }
    TreeNode head = helper(num, 0, num.length - 1);
    return head;
}

public TreeNode helper(int[] num, int low, int high) {
    if (low > high) { 
        return null;
    }
    int mid = (low + high) / 2;
    TreeNode node = new TreeNode(num[mid]);
    node.left = helper(num, low, mid - 1);
    node.right = helper(num, mid + 1, high);
    return node;
}