Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example:
Given the sorted array: [-10,-3,0,5,9],
One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:
0
/ \
-3 9
/ /
-10 5


思路:

其实就是构建一个二叉搜索树，对于搜索树，有三个特点:

1、根节点左边的都比根节点小。

2、根节点右边的都比根节点大。

3、左孩子和右孩子都是搜索树。

因此通过递归构建左孩子，右孩子。

class Solution(object):
def sortedArrayToBST(self, nums):
"""
:type nums: List[int]
:rtype: TreeNode
"""
if len(nums) == 1:
return TreeNode(nums[0])
elif len(nums) == 0:
return None
else:
mid = (len(nums)-1) / 2
node = TreeNode(nums[mid])
node.left = self.sortedArrayToBST(nums[:mid])
node.right = self.sortedArrayToBST(nums[mid+1:])
return node