Based on 108. Convert Sorted Array to Binary Search Tree. Basically just convert the sorted linked list `head`

into a sorted array `nums`

and then build the tree. It's O(n) in space but it's concise and fast. Runtime is ~260ms on OJ.

```
def sortedListToBST(self, head):
def helper(L, R):
if L <= R:
mid = L+(R-L+1)/2
root = TreeNode(nums[mid])
root.left = helper(L, mid-1)
root.right = helper(mid+1, R)
return root
nums, node = [], head
while node:
nums.append(node.val)
node = node.next
return helper(0, len(nums)-1)
```