[LeetCode&Python] Problem 108. 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

# 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
        """
        def bst(n,r):
            l=len(n)
            if l==1:
                if n[0]<=r.val:
                    r.left=TreeNode(n[0])
                else:
                    r.right=TreeNode(n[0])
            elif l>1:
                mid=l//2
                r2=TreeNode(n[mid])
                if r.val>=r2.val:
                    r.left=r2
                else:
                    r.right=r2
                bst(n[:mid],r2)
                bst(n[mid+1:],r2)
        if nums:
            ln=len(nums)
            root=TreeNode(nums[ln//2])
            bst(nums[:ln//2],root)
            bst(nums[ln//2+1:],root)
            return root
        else:
            return nums