108.将有序数组转换为二叉搜索树.py

#
# @lc app=leetcode.cn id=108 lang=python
#
# [108] 将有序数组转换为二叉搜索树
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# https://leetcode-cn.com/problems/convert-sorted-array-to-binary-search-tree/description/
#
# algorithms
# Easy (67.32%)
# Likes:    255
# Dislikes: 0
# Total Accepted:    34.2K
# Total Submissions: 50.8K
# Testcase Example:  '[-10,-3,0,5,9]'
#
# 将一个按照升序排列的有序数组，转换为一棵高度平衡二叉搜索树。
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# 本题中，一个高度平衡二叉树是指一个二叉树每个节点 的左右两个子树的高度差的绝对值不超过 1。
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# 示例:
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# 给定有序数组: [-10,-3,0,5,9],
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# 一个可能的答案是：[0,-3,9,-10,null,5]，它可以表示下面这个高度平衡二叉搜索树：
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# ⁠     0
# ⁠    / \
# ⁠  -3   9
# ⁠  /   /
# ⁠-10  5
# 
# 
#

# @lc code=start
# 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 helper(nums,l,r):
            if r<l:
                return None
            mid=int(l+(r-l)/2)
            root=TreeNode(nums[mid])
            root.left=helper(nums,l,mid-1)
            root.right=helper(nums,mid+1,r)
            return root
        return helper(nums,0,len(nums)-1) if nums else None
# @lc code=end