I don't understand why my method is not passing test case 32?

```
def sortedArrayToBST(self, nums):
"""
:type nums: List[int]
:rtype: TreeNode
"""
if nums:
n = len(nums)
count = 0
return self.toBSTHelper(nums, 0, n-1 ,count)
else:
return None
def toBSTHelper(self, nums, i, j, count):
count += 1
if count == 10:
return None
if i== j:
return TreeNode(nums[j])
elif i > j:
return None
mid = i+((j-i)//2)
root = TreeNode(nums[mid])
root.left = self.toBSTHelper(nums, i, mid-1, count)
root.right = self.toBSTHelper(nums, mid+1,j, count)
return root
```