方法一：
i=sizeof(int)*8;
scanf("%d",&a);
int j = a>>i&1;
for(;i>0;i--,printf("%d",a>>i&1));
方法二：
long dnum;
char * cnum;
cnum = new char[100];
printf("please input a decimal number:\n");
scanf("%ld",&dnum);
ltoa(dnum,cnum,2);
printf("the binary number is:%s",cnum);
方法三：
while(index>0)
{
a[i]=index % 2;
index=index/2;
i--;
i=sizeof(int)*8;
scanf("%d",&a);
int j = a>>i&1;
for(;i>0;i--,printf("%d",a>>i&1));
方法二：
long dnum;
char * cnum;
cnum = new char[100];
printf("please input a decimal number:\n");
scanf("%ld",&dnum);
ltoa(dnum,cnum,2);
printf("the binary number is:%s",cnum);
方法三：
while(index>0)
{
a[i]=index % 2;
index=index/2;
i--;