leetcode 669. 修剪二叉搜索树

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
        data =root.val
        if data<L or data>R:
            a=self.trimBST(root.left,L,R)
            b= self.trimBST(root.right,L,R)
            return  a if b is None else b

        root.left=self.trimBST(root.left, L, R)
        root.right=self.trimBST(root.right, L, R)

        return root