LeetCode108 Convert Sorted Array to Binary Search Tree

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>想要一个平衡二叉树就用递归的方法，每次取中间的点为root，左边为左子树，右边为右子树。 

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def sortedArrayToBST(self, nums):
        """
        :type nums: List[int]
        :rtype: TreeNode
        """    
        return self.dfs(nums)
    
    def dfs(self,nums):
        
        if len(nums) == 0:
            return         
        mid = len(nums) // 2
        root = TreeNode(nums[mid])
        root.left = self.dfs(nums[:mid])
        root.right = self.dfs(nums[mid+1:])
        
        return root