669. Trim a Binary Search Tree

Medium

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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.

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 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

# 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 or (root.left is None and root.right is None and (root.val < low or root.val > high)):
            return None
        if root.val < low:
            return self.trimBST(root.right, low, high)
        elif root.val > high:
            return self.trimBST(root.left, low, high)
        # Check for left value
        if root.left and root.left.val < low:
            root.left = self.trimBST(root.left.right, low, high)
        else:
            root.left = self.trimBST(root.left, low, high)
        # Check for right value
        if root.right and root.right.val > high:
            root.right = self.trimBST(root.right.left, low, high)
        else:
            root.right = self.trimBST(root.right, low, high)
        return root
            
            
         

More Clean Solution

If you will check above solution you might notice that Line 17 & 22 are being handled by Line 12 & 14 respectively.

class Solution:
    def trimBST(self, root: Optional[TreeNode], low: int, high: int) -> Optional[TreeNode]:
        if not root:
            return None
        if root.val < low:
            return self.trimBST(root.right, low, high)
        elif 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