题目：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.

题目分析：对于递归而言，是指从某个模式来不停的返回，想好要每一步的操作，以根节点作为模式的触发点，也就是从根的模式来进行思考，考虑每一步是什么，然后返回什么比较合计。
还要利用搜索树的性质来进行处理。

python代码的实现：

class Solution(object):
    def trimBST(self, root, L, R):
        """
        :type root: TreeNode
        :type L: int
        :type R: int
        :rtype: TreeNode
        """
        if root == None:
            return None
        if root.val < L:
            node = self.trimBST(root.right, L, R)
            root = node
        elif root.val > R:                                          
            node = self.trimBST(root.left, L, R)
            root = node
        else:                           
            node = self.trimBST(root.left, L, R)
            root.left = node
            node = self.trimBST(root.right, L, R) 
            root.right = node
        return root