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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