LeetCode669

 本题注意点：if root.val<L，直接return即可

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

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
        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

        # version2：（我）
        # if root is None:    return None
        # while(root.val < L or root.val > R):
        #     if root.val < L:
        #         root = root.right
        #     if root.val > R:
        #         root = root.left
        # trimBST(root.left, L, R)
        # trimBST(root.right, L, R)
        # return root