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　题目描述：

 

 方法一：O(n) O(n)

class Solution:    def sortedListToBST(self, head: ListNode) -> TreeNode:        nums = []        while head:            nums.append(head.val)            head = head.next        def helper(nums):            if not nums:                return None            m = len(nums)//2            root = TreeNode(nums[m])            root.left = helper(nums[0:m])            root.right = helper(nums[m+1:])            return root        return helper(nums)

方法二：O(nlogn)O(logn)

class Solution:    def findMid(self,head):        prevPtr = None        slowPtr = head        fastPtr = head        while fastPtr and fastPtr.next:            prevPtr = slowPtr            slowPtr = slowPtr.next            fastPtr = fastPtr.next.next        if prevPtr:            prevPtr.next = None        return slowPtr    def sortedListToBST(self, head: ListNode) -> TreeNode:        if not head:            return None        mid = self.findMid(head)        node = TreeNode(mid.val)        if head==mid:            return node        node.left = self.sortedListToBST(head)         node.right = self.sortedListToBST(mid.next)         return node

 

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