Trim a Binary Search Tree

根据BST的性质，大于R的右半边可以直接砍掉，小于L的左半边可以直接砍掉

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