OJ Board

#include<cstdio>
#include<cstdlib>
#include<cstring>
int main()
{
long long int k,i,j,d,c,n;
char a[1000000];
while(scanf("%lld",&n)==1&&n!=0)
{
c=0;
j=0;
while(n!=0)
{
d=n%2;
a[j]=d+48;
n=n/2;
j++;
}

for(i=0;a !=NULL;i++)
{
if(a =='1')
{
c++;
}
}
strrev(a);
printf("The parity ...

#include<stdio.h>
int main()
{
long n,i,j,a[50];
while(1)
{
scanf("%ld",&n);
if(n<=0) break;

for(i=0;n>0;i++)
{
a =n%3;
n=n/3;
}
for(j=i-1;j>=0;j--)
{
printf("%ld",a[j]);
}
printf("\n");
}
return 0;
}
whats the problem with this code?