109. Convert Sorted List to Binary Search Tree

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

Code:

# 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):
    if not head:
        return 
    if not head.next:
         return TreeNode(head.val)
    slow, fast = head, head.next.next
    while fast and fast.next:
        fast = fast.next.next
        slow = slow.next
    # tmp points to root
    tmp = slow.next
    # cut down the left child
    slow.next = None
    root = TreeNode(tmp.val)
    root.left = self.sortedListToBST(head)
    root.right = self.sortedListToBST(tmp.next)
    return root