## 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``````

## 解法一

### 思路

``````def trimBST(self, root, L, R):
if not root:
return root

# TODO

root.left = self.trimBST(root.left, L, R)
root.right = self.trimBST(root.right, L, R)
return root``````

``````if root.val < L:
return self.trimBST(root.right, L, R)
if root.val > R:
return self.trimBST(root.left, L, R)``````

### Python

``````# 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 root
if root.val < L:
return self.trimBST(root.right, L, R)
if root.val > R:
return self.trimBST(root.left, L, R)
root.left = self.trimBST(root.left, L, R)
root.right = self.trimBST(root.right, L, R)
return root``````

### Java

``````/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
class Solution {
public TreeNode trimBST(TreeNode root, int L, int R) {
if (root == null) {
return root;
}
if (root.val < L) {
return trimBST(root.right, L, R);
}
if (root.val > R) {
return trimBST(root.left, L, R);
}
root.left = trimBST(root.left, L, R);
root.right = trimBST(root.right, L, R);
return root;
}
}``````

### C++

``````/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
TreeNode *trimBST(TreeNode *root, int L, int R) {
if (root == nullptr) {
return root;
}
if (root->val < L) {
return trimBST(root->right, L, R);
}
if (root->val > R) {
return trimBST(root->left, L, R);
}
root->left = trimBST(root->left, L, R);
root->right = trimBST(root->right, L, R);
return root;
}
};``````