递归。每次数组的中点元素为root, 再递归地建立左右子树。

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param num, a list of integers
    # @return a tree node
    def sortedArrayToBST(self, num):
        return self.createBST(num, 0, len(num) - 1)
    
    def createBST(self, num, start, end):
        if start > end: return None
        mid = (start + end) / 2
        root = TreeNode(num[mid])
        root.left = self.createBST(num, start, mid - 1)
        root.right = self.createBST(num, mid + 1, end)
        return root