leetcode:先序遍历构造二叉树(python)

乖乖的函数 2020-11-13 02:53:38
LeetCode 二叉树 构造 遍历 二叉


1. 题目描述

返回与给定先序遍历 preorder 相匹配的二叉搜索树(binary search tree)的根结点。

(回想一下,二叉搜索树是二叉树的一种,其每个节点都满足以下规则,对于 node.left 的任何后代,值总 < node.val,而 node.right 的任何后代,值总 > node.val。此外,先序遍历首先显示节点的值,然后遍历 node.left,接着遍历 node.right。)

示例:
输入:[8,5,1,7,10,12]
输出:[8,5,10,1,7,null,12]

在这里插入图片描述

提示:
1 <= preorder.length <= 100
先序 preorder 中的值是不同的。

2. 思路

利用递归,关键是找到左子树和右子树的先序遍历。因为是二叉搜索树,因此左子树的值比根节点的值要小,右子树的值要比根节点的值要大。因为是前序遍历,因此数组的第一个节点就是根节点,从遍历顺序的第二个值开始遍历,直到大于根节点的值出现这一段就是左子树的先序遍历顺序,剩下的就是右子树的先序遍历顺序。递归。

2.1 代码

class Solution:
def bstFromPreorder(self, preorder: List[int]) -> TreeNode:
if len(preorder) == 0:
return None
root = TreeNode(preorder[0])
i = 1
length = len(preorder)
while i < length and preorder[i] < preorder[0] :
i += 1
left = preorder[1:i]
right = preorder[i:]
root.left = self.bstFromPreorder(left)
root.right = self.bstFromPreorder(right)
return root
版权声明
本文为[乖乖的函数]所创,转载请带上原文链接,感谢
https://blog.csdn.net/ggdhs/article/details/93592716

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