正確程式碼:

class Solution:
    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
 

#錯題分析
class Solution:
    def trimBST(self, root, L, R):
        """
        :type root: TreeNode
        :type L: int
        :type R: int
        :rtype: TreeNode
        """
        root = self.fun_2(root, L, R)
        if root is None:
            return root
        self.func_1(root, root.left, L, R)
        self.func_1(root, root.right, L, R)
        return root

    #找根節點    
    def func_1(self, parent, node, L, R):
        if node is None:
            return
        elif node.val>R:
            parent.right = node.left
        elif node.val<L:
            parent.left = node.right
                   
        self.func_1(node, node.left, L, R)
        self.func_1(node, node.right, L, R)

    #進行裁剪
    def fun_2(self, node, L, R):
        if L<=node.val<=R or node is None:
            return node
        elif node.val>R:
            node = node.left#在這個地方出現問題,缺少了遞迴..
        elif node.val<L:
            node = node.right
        return self.fun_2(node, L, R)
 

ps:好菜啊!