# Leetcode: Trim a Binary Search Tree

Trim a Binary Search Tree

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
```

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

```## Blog link: https://code.dennyzhang.com/trim-a-binary-search-tree
## Basic Ideas: recursive pre-order traseveral
## Complexity:
## Assumptions: No duplicate elements in the BST

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

class Solution(object):
def trimBST(self, root, L, R):
"""
:type root: TreeNode
:type L: int
:type R: int
:rtype: TreeNode
"""
##
if root is None:
return None

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

if value < L:
# left tree won't be qualified
return self.trimBST(root.right, L, R)

if value > R:
# right tree won't be qualified
return self.trimBST(root.left, L, R)
```

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