为什么我这代码还是超时

要求：时间复杂度为O（n）,空间复杂度为O（1）。
一下是我的代码：

include <stdio.h>

int main()
{
int a,b,c,d,i;
char ch;
a=b=c=d=0;
while((ch=getchar())!='\n')
{
if(ch=='a') a++;
else if(ch=='b') b++;
else if(ch=='c') c++;
else if(ch=='d') d++;
}
for(i=0;i<a;i++) printf("a");
for(i=0;i<b;i++) printf("b");
for(i=0;i<c;i++) printf("c");
for(i=0;i<d;i++) printf("d");
printf("\n");
return 0;
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