按分段统计学生成绩：要求按 90－100、80－89、70－79、60－69、60以下五档分别统计各分数段人数，输入为负数时结束。（分别用if-else和switch）-Segmented by student achievement statistics: 90-100,80-89,70-79,60-69,60 following five files required by the statistical numbers are the fraction of the input end o