109. 有序链表转换二叉搜索树

和题目108类似：108是数组  https://www.cnblogs.com/MarkLeeBYR/p/16906818.html

public TreeNode sortedListToBST(ListNode head) {
    if (head == null) {
        return null;
    }
    return helper(head, null);
}
private TreeNode helper(ListNode head, ListNode tail){
    if (head == tail) {
        return null;
    }
    ListNode fast = head;
    ListNode slow = head;
    while (fast != tail && fast.next != tail) {
        fast = fast.next.next;
        slow = slow.next;
    }
    TreeNode root = new TreeNode(slow.val);
    root.left = helper(head, slow);
    root.right = helper(slow.next, tail);
    return root;
}