Abstract: The paper discusses the Web proxy location problem with consideration of both read and update operations to the data on the Internet when controlling the distance between the Web proxies and the server and finds the optimal number of proxies and their placement，such that the overall access cost is minimized.An algorithm with time complexity O（n²） is gotten which uses binary tree and the dynamic programming，where n is the number of nodes in the net.

Key words: proxy, binary tree, dynamic programming, delay restraint

摘要： 该文考虑网络数据更新，需要控制代理服务器与客户的距离时，网络中的代理服务器的放置问题。找到代理服务器的最优数量和放置位置，使网络中数据访问的总花费（包括数据读取和更新）最小。利用二叉树结构和动态规划方法，得到了一个时间复杂度O（n²）的多项式时间算法，其中n为网络结点数。

关键词: 代理服务器, 二叉树, 动态规划, 时延约束