669. Trim a Binary Search Tree¶
Edited by Charles
Medium · Python · Binary Tree
Problem
Read the original problem on LeetCode.
The official page contains the problem statement, examples, and constraints.
Explanation, examples & other approaches
Core idea
The core idea of the solution is, even though the value of the root is less than low, the right subtree may still meet the requirement; similarly, even though the value of the root is greater than high, the left subtree may still work.
Original study notes follow, including intermediate snippets and earlier attempts.
Note:
The core idea of the solution is, even though the value of the root is less than low, the right subtree may still meet the requirement; similarly, even though the value of the root is greater than high, the left subtree may still work.
Study record
| Field | Value |
|---|---|
| Attempts | 1 |
| Mastery | ❚ |
| The Latest Attempt | January 21, 2026 |
Python solution¶
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def trimBST(self, root: Optional[TreeNode], low: int, high: int) -> Optional[TreeNode]:
if not root:
return None
if root.val > high:
return self.trimBST(root.left, low, high)
if root.val < low:
return self.trimBST(root.right, low, high)
root.left = self.trimBST(root.left, low, high)
root.right = self.trimBST(root.right, low, high)
return root