108.py

'''
	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 constructTree(nums, start, end):
        	if start > end:
        		return None

        	mid = (start+end)/2
        	node = TreeNode(nums[mid])

        	if start == end:
        		return node

        	node.left = constructTree(nums, start, mid-1)
        	node.right = constructTree(nums, mid+1, end)
        	return node

        return constructTree(nums, 0, len(nums)-1)