669. Trim a Binary Search Tree 修剪二叉搜索树

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

给定二叉搜索树，将最低和最高边界作为L和R，修剪树，使其所有元素位于[L，R]（R> = L）中。您可能需要更改树的根，所以结果应该返回修剪的二叉搜索树的新根。

class Solution(object):
    def trimBST(self, root, L, R):
        """
        :type root: TreeNode
        :type L: int
        :type R: int
        :rtype: TreeNode
        """
        if not root:
            return root
        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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