# 669. Trim a Binary Search Tree 修剪二叉搜索树

@TOC

## # 题目描述

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.

Example 1:

``````Input:
1
/ \
0   2

L = 1
R = 2

Output:
1
\
2
``````

Example 2:

``````Input:
3
/ \
0   4
\
2
/
1

L = 1
R = 3

Output:
3
/
2
/
1
``````

## # 解题方法

### # 递归

``````# 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, L, R):
"""
:type root: TreeNode
:type L: int
:type R: int
:rtype: TreeNode
"""
if not root:
return None
if root.val > R:
return self.trimBST(root.left, L, R)
elif root.val < L:
return self.trimBST(root.right, L, R)
else:
root.left = self.trimBST(root.left, L, R)
root.right = self.trimBST(root.right, L, R)
return root
``````

## # 日期

2018 年 11 月 8 日 —— 项目进展缓慢