109. 有序链表转换二叉搜索树

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        if(head == null) return null;
        return help(head,null);
    }
    private TreeNode help(ListNode head,ListNode tail){
        if(head == tail) return null;
        ListNode fa = head;
        ListNode sl = head;
        //循环条件
        while(fa != tail && fa.next != tail){
            fa = fa.next.next;
            sl = sl.next;
        }
        TreeNode root = new TreeNode(sl.val);
        root.left = help(head,sl);
        root.right = help(sl.next,tail);
        return root;
    }
}