[LeetCode] Challenge log 669

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669. Trim a Binary Search Tree

Recursive solution: Use the special property of binary search tree: left sons are smaller while right sons are greater.

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:

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Input: 
    1
   / \
  0   2

  L = 1
  R = 2

Output: 
    1
      \
       2

Example 2:

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Input: 
    3
   / \
  0   4
   \
    2
   /
  1

  L = 1
  R = 3

Output: 
      3
     / 
   2   
  /
 1
Soulution:
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class Solution:
    def trimBST(self, root, L, R):
        """
        :type root: TreeNode
        :type L: int
        :type R: int
        :rtype: TreeNode
        """
        if root is None: return None
        if root.val < L: return self.trimBST(root.right, L, R)
        if root.val > R: return self.trimBST(root.left, L, R)
        
        root.left = self.trimBST(root.left, L, R)
        root.right = self.trimBST(root.right, L, R)
        
        return root
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