const char* strstr(const char* s1,const char* s2)//找s2在s1中第一次出现的位置,
{
assert(s2&&s1);
const char* p = s1,*r = s2;
while (*p!='/0')
{
while (*p+===*r++);
if (*r=='/0')
{
return p;
}
else
{
r = s2;
p = ++s1;
}
}
return NULL;
}