题目要求
Given a binary search tree and the lowest and highest boundaries as L
and R
, trim the tree so that all its elements lies in [L, R]
(R >= L). You might need to change the root of the tree, so the result should return the new root of the trimmed binary search tree.
题目分析及思路
题目给出一棵二叉搜索树和一个上下界,要求对树剪枝,使得结果树中的所有结点的值都在给定的区间内。二叉搜索树的定义为:该树可为空;不空的话,若它的左子树不空,则左子树上所有结点的值均小于它的根结点的值,若它的右子树不空,则右子树上所有结点的值均大于它的根结点的值。根据该定义,我们可以使用递归的方法,当root的值位于L
和R
之间,则递归修剪其左右子树,返回root。当root的值小于L
,则其左子树的值都小于L
,抛弃左子树,返回修剪过的右子树。当root的值大于R
,则其右子树的值都大于R
,抛弃右子树,返回修剪过的左子树。
python代码
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def trimBST(self, root: TreeNode, L: int, R: int) -> TreeNode:
if not root:
return
if root.val < L:
return self.trimBST(root.right, L, R)
elif root.val > R:
return self.trimBST(root.left, L, R)
else:
root.left = self.trimBST(root.left, L, R)
root.right = self.trimBST(root.right, L, R)
return root