Swift - 高阶函数介绍（map、flatMap、filter、reduce）
https://blog.csdn.net/Alexander_Wei/article/details/78543410?spm=1001.2101.3001.6650.3&utm_medium=distribute.pc_relevant.none-task-blog-2%7Edefault%7ECTRLIST%7ERate-3-78543410-blog-127072873.235%5Ev38%5Epc_relevant_anti_t3_base&depth_1-utm_source=distribute.pc_relevant.none-task-blog-2%7Edefault%7ECTRLIST%7ERate-3-78543410-blog-127072873.235%5Ev38%5Epc_relevant_anti_t3_base&utm_relevant_index=6