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Trim a Binary Search Tree

Problem Statement​

Given the root of a binary search tree and the lowest and highest boundaries as low and high, trim the tree so that all its elements lies in [low, high]. Trimming the tree should not change the relative structure of the elements that will remain in the tree (i.e., any node's descendant should remain a descendant). It can be proven that there is a unique answer.

Return the root of the trimmed binary search tree. Note that the root may change depending on the given bounds.

Leetcode Link

Example 1:​

Input: root = [1,0,2], low = 1, high = 2
Output: [1,null,2]

Example 2:​

Input: root = [3,0,4,null,2,null,null,1], low = 1, high = 3
Output: [3,2,null,1]

Constraints:​

  • The number of nodes in the tree is in the range [1, 104].
  • 0 <= Node.val <= 104
  • The value of each node in the tree is unique.
  • root is guaranteed to be a valid binary search tree.
  • 0 <= low <= high <= 104

Code​

Python Code

class Solution:
    def trimBST(self, root: Optional[TreeNode], low: int, high: int) -> Optional[TreeNode]:
        if root == None: return None
        if root.val < low:
            return self.trimBST(root.right, low, high)
        if root.val > high:
            return self.trimBST(root.left, low, high)

        root.left = self.trimBST(root.left, low, high)
        root.right = self.trimBST(root.right, low, high)
        return root