翻转

有一个字符数组的内容为:"student a am i",
请你将数组的内容改为"i am a student".
要求：
不能使用库函数。
只能开辟有限个空间（空间个数和字符串的长度无关）。

student a am i
i ma a tneduts
i am a student
 

[code]
#include <stdio.h>
#include <string.h>
#include <assert.h>
char * fanzhuan(char *str)
{
assert(str);
char *l = str;
char *r = str;
while(*r != '\0')
{
r++;
}
while(l < r-1)
{
char tmp = *l;
*l = *(r-1);
*(r-1) = tmp;
l++;
r--;
}
l = str;
r = str;
while(*r != '\0')
{
while(*r != 32 && *r != '\0')
{
r++;
}
char *p = r;
while(l < r-1)
{
char tmp = *l;
*l = *(r-1);
*(r-1) = tmp;
l++;
r--;
}
if(*p == '\0')
{
return str;
}
l = p+1;
r = p+1;
}
return str;
}

int main ()
{
char a[] = "student a am i";
char *ret = fanzhuan(a);
printf("%s\n",ret);
return 0;
}