convert-sorted-list-to-binary-search-tree.py

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def sortedListToBST(self, head: ListNode) -> TreeNode:
        if not head: return
        if not head.next: return TreeNode(head.val)
        slow, fast, pre = head, head, None
        while fast and fast.next:
            pre, slow, fast = slow, slow.next, fast.next.next
        pre.next = None   # cut off the left half

        root = TreeNode(slow.val)
        root.left = self.sortedListToBST(head)
        root.right = self.sortedListToBST(slow.next)
        return root