这里高度平衡的二叉树，核心思想就是找中位数！

这里用到一种非常重要的解题方案：切割链表【万金油代码】

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【万金油代码】 -> 用来切割链表

class Solution:
    def find_mid(self, head):
        pre = None
        slow = head
        fast = head
        while fast and fast.next:
            pre = slow
            slow = slow.next
            fast = fast.next.next
        if pre:
            pre.next = None
        return slow
    def sortedListToBST(self, head: ListNode) -> TreeNode:
        if not head:
            return
        mid = self.find_mid(head)
        root = TreeNode(mid.val)
        if head == mid:
            return root
        root.left = self.sortedListToBST(head)
        root.right = self.sortedListToBST(mid.next)
        return root