108 Convert Sorted Array to Binary Search Tree

class Solution:
    # @param {integer[]} nums
    # @return {TreeNode}
    def sortedArrayToBST(self, nums):
        return self.help(nums, 0, len(nums) - 1)

    def help(self, nums, left, right):
        if left == right:
            return TreeNode(nums[left])
        if left > right:
            return None
        mid = (left + right) / 2
        node = TreeNode(nums[mid])
        node.left = self.help(nums, left, mid - 1)
        node.right = self.help(nums, mid + 1, right)
        return node