【leetcode】109.Convert Sorted List to Binary Search Tree【java】わかりやすい方法

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

/**
 * 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; }
 * }
 */
//      ，       ，       ，           ，             ，        。
//                        
public class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        return helper(head, null);
    }
    public TreeNode helper(ListNode head, ListNode tail) {
        if (head == null || head == tail){
            return null;
        }
        ListNode slow = head;
        ListNode fast = head;
        while (fast.next != tail && fast.next.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;
    }
}