19.leetcode108_convert_sorted_array_to_binary_search_tree

1.题目描述

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.

给出一段有序列表，返回一个平衡高度的二叉树

2.题目分析

首先确定二叉树的首节点，它的数值即为列表的中间元素，列表长度为双数时，为列表右边靠近中心的第一个元素。然后按三个节点构成一个树杈的方法考虑。循环遍历列表，只将当前列表中心及其左右元素构成一个树杈。

3.解题思路

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

class Solution:
    
    def sortedArrayToBST(self, nums):
        """
        :type nums: List[int]
        :rtype: TreeNode
        """
        len_nums=len(nums)
        if len_nums==0: 
            return None
        elif len_nums==1:
            return TreeNode(nums[0])
        else:
            center=int(len_nums/2) 
            node=TreeNode(nums[center])
            node_left=nums[:center]
            node_right=nums[center+1:len_nums]
            node.left=self.sortedArrayToBST(node_left)
            node.right=self.sortedArrayToBST(node_right)
            return node