leetcode: 109. Convert Sorted List to Binary Search Tree

Difficulty

Medium.

Problem

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

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example:

Given the sorted linked list: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

      0
     / \
   -3   9
   /   /
 -10  5

AC

class Solution():
    def sortedListToBST(self, head):
        if not head:
            return None
        if not head.next:
            root, root.left, root.right = TreeNode(head.val), None, None
            return root
        preslow = slow = fast = head
        while fast and fast.next:
            preslow, slow, fast = slow, slow.next, fast.next.next
        preslow.next = None
        root = TreeNode(slow.val)
        root.left = self.sortedListToBST(head)
        root.right = self.sortedListToBST(slow.next)
        return root

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