编程实现返回不大于某值的最大的2次幂

//其本质即找到该值二进制形式中最高位的1，其余位都置为0

unsigned long Max2(unsigned long x)
{
 unsigned long y=x;
 while(x)
 {
  y=x;
  x=x&(x-1);
 }
 return y;
}
unsigned long Max2_2(unsigned long x)
{
 unsigned long y=1;
 while(x!=1)
 {
  x=x>>1;
  y=y<<1;
 }
 return y;
}
unsigned long Max2_3(unsigned long x)
{
 char str[20];
 memset(str,0,20);
 ultoa(x,str,2);
 char *p=str;
 while(*p)
 {
  if(*p=='1')
   break;
  ++p;
 }
 int y=1;
 while(*p)
 {
  y=y<<1;
  ++p;
 }
 return y>>1;
}