Convert Sorted Array to Binary Search Tree

There are two ways to deal with array indexes.

# 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 sortedArrayToBST(self, nums):
        """
        :type nums: List[int]
        :rtype: TreeNode
        """
        if nums == None or len(nums) == 0:
            return None
        return self.sortedArrayToBSTHelper(nums, 0, len(nums) - 1)
    def sortedArrayToBSTHelper(self, nums, start, end):
        if start > end:
            return None
        mid = (start + end) / 2
        root = TreeNode(nums[mid])
        left = self.sortedArrayToBSTHelper(nums,start, mid-1)

        right = self.sortedArrayToBSTHelper(nums, mid+1, end)

        root.left = left
        root.right = right

        return root