Removing Duplicate values and adding 0 on their place - c

Question

I have written a code to remove duplicate values and add 0 on there place .
But i feel like my code should be much better than this,if anybody can give a better idea of developing this code.
Please suggest me and advice me.
Input--2,3,4,3,6
output--2,3,4,0,6
Here is my code:
#include<stdio.h>
int main()
{
int a[100],b[100];
int i,j,size;
scanf("%d",&size);
for(i=0;i<size;i++)
{
scanf("%d",&a[i]);
}
for(i=0;i<size;i++)
{
b[i]=a[i];
}
for(i=0;i<size;i++)
{
for(j=i+1;j<size;j++)
{
if(a[i]==a[j])
{
b[j]=0;
}
}
}
for(i=0;i<size;i++)
printf("%d\n",b[i]);
return 0;
}

Answer 1

Clear duplicates as they are entered as follows, comparing with the values entered so far:
#include<stdio.h>
int main() {
int a[100];
int i,j,size;
scanf("%d",&size);
for(i=0;i<size;i++)
{
scanf("%d",&a[i]);
for(j=0;j<i;j++){
if(a[j]==a[i]) {
a[i]=0; /* found duplicate among previous entries! */
break;
}
}
}
for(i=0;i<size;i++)
printf("%d\n",a[i]);
return 0;
}

Answer 2

Here is some of my idea:
Solution 1:
Use bit to indicate whether a number occurred yet.
e.g On 32bit machine, 1 int has 32bit, if your number range is 1 ~ 1000, then you need 32 int, you can change it range when you met a larger number, by realloc().
If your number range is small, then it's quite suitable.
Solution 2:
Store sorted numbers in a binary tree, so that you can search quicker.

Answer 3

You can use the single array itself, and mark at the positions where you find a duplicate in it. Something like this.
#include<stdio.h>
int main() {
int a[100];
int i,j,size;
scanf("%d",&size);
for(i=0;i<size;i++) {
scanf("%d",&a[i]);
}
for(i=1; i<size; i++) {
for(j=i-1; j>=0; j--) {
if(a[i]==a[j]) {
a[i]=0;
break;
}
}
}
for(i=0;i<size;i++)
printf("%d ",a[i]);
return 0;
}

Answer 4

Here's an improvement, but I agree it feels like it can be significantly better (Maybe I'll get back to this later...):
#include<stdio.h>
int main()
{
int a[100];
int i,j,size,left;
scanf("%d",&size);
for(i=0;i<size;i++)
{
scanf("%d",&a[i]);
}
left = size;
for(i=0;i<size&&left>1;i++) // If there's only 1 left, it's not a duplicate
{
if(a[i] == 0) // No need to test these, already done
continue;
for(j=i+1,left=0;j<size;j++)
{
if(a[i]==a[j])
{
a[j]=0;
}
if(a[j]!=0)
left++; // If we don't get here, there's nothing left to test
}
}
for(i=0;i<size;i++)
printf("%d\n",a[i]);
return 0;
}
So basically, don't search for 0's ahead of current position, and when searching for anything else count (mark actually) if there is anything left to test.