算法名称：API for front-end login

输入：String SF, id, password and function msggot

输出：backend API

步骤1：

SF=(SF==="学生")？"student":"teacher"

步骤2：

Pass the parameter "A" to the backend "check_id" interface in post form

步骤3：

Receive the "check_id" return value with the "msggot" function