func extract3(c chan task) []task {
值得注意的是，针对此前备受用户吐槽的「微信文件重复保存吃内存」问题，微信官方此次也借机作出了明确回应：
，详情可参考safew官方下载
Shortcut Pre-calculation: For the most commonly used speed profiles, the travel costs (time/distance) for shortcuts between border points within each cluster are pre-calculated and stored. (Each border point effectively has an "entry" and "exit" aspect for directed travel).
思路：① 算每辆车到达时间 = (target - position) / speed；② 按位置降序排序；③ 单调递增栈：仅当当前时间 栈顶时间时压栈（新车队），否则合并。栈长即为车队数。可优化为用变量代替栈。