109. Convert Sorted List to Binary Search Tree

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

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

class Solution(object):
    def sortedListToBST(self, head):
        """
        :type head: ListNode
        :rtype: TreeNode
        """
        if not head: return None

        if not head.next: return TreeNode(head.val)
        slow,fast=head,head.next.next
        while fast and fast.next:
            slow=slow.next
            fast=fast.next.next
        root=TreeNode(slow.next.val)
        print 'created node',root.val
        root.right=self.sortedListToBST(slow.next.next)
        slow.next=None
        root.left=self.sortedListToBST(head)
        return root