代码提交上去之后说是超时，可我这种写法我不太明白怎么把时间复杂度降低。下面是题目以及我写的代码，希望大家能帮忙解决下，谢谢了。
Problem Description
Each course grade is one of the following five letters: A, B, C, D, and F. (Note that there is no grade E.) The grade A indicates superior achievement , whereas F stands for failure. In order to calculate the GPA, the letter grades A, B, C, D, and F are assigned the following grade points, respectively: 4, 3, 2, 1, and 0.
Input
The input file will contain data for one or more test cases, one test case per line. On each line there will be one or more upper case letters, separated by blank spaces.
Output
Each line of input will result in exactly one line of output. If all upper case letters on a particular line of input came from the set {A, B, C, D, F} then the output will consist of the GPA, displayed with a precision of two decimal places. Otherwise, the message "Unknown letter grade in input" will be printed.
Sample Input
A B C D F
B F F C C A
D C E F
Sample Output
2.00
1.83
Unknown letter grade in input
代码：
#include
#include
int main()
{
float sum;
float count;
char ch;
int flag;
while(1)
{
sum=0;
count=0;
flag=1;
while((ch=getchar())!='\n')
{
switch(ch)
{
case'A':sum=sum+4;count++;break;
case'B':sum+=3;count++;break;
case'C':sum+=2;count++;break;
case'D':sum+=1;count++;break;
case'F':sum+=0;count++;break;
case'E':flag=0;break;
case' ':break;
}
}
if(flag)
{
printf("%.2f\n",sum/count);
}
else
{
printf("Unknown letter grade in input\n");
}
}
return 0;
}
