[二叉搜索树]二叉搜索树的下一个节点

        self.last = None
        self.res = None
        def dfs(root):
            if not root:
                return
            
            dfs(root.left)
            if self.last == pNode:
                self.res = root
            self.last = root
            dfs(root.right)

        dfs(root)
        return self.res