有一个学生类student，包括学生姓名、成绩，设计一个友元函数，比较两个学生成绩的高低，并求出最高分和最低分的学生。--Have a student category of student, including student names, achievements, the design of a friend function to compare two of the high and low student achievement, and obtained the highest points and lowest points of the students.