i used a array .in that array value increment according to id. after all inputs it is checking at with index value is grater than 2.
int a[50001];
#include<stdio.h>
int main(){
int n1,n2,n3,i,n,count=0;
scanf("%d %d %d",&n1,&n2,&n3);
if((n1<0 || n1>50000)||(n2<0 || n2>50000)||(n3<0 || n3>50000)){
exit(1);
}
for(i=1;i<=50000;i++){
a[i]=0;
}
for(i=1;i<=n1;i++){
scanf("%d",&n);
a[n]++;
}
for(i=1;i<=n2;i++){
scanf("%d",&n);
a[n]++;
}
for(i=1;i<=n3;i++){
scanf("%d",&n);
a[n]++;
}
for(i=1;i<=50000;i++){
if(a[i]>=2){
count++;
}
}
printf("%d\n",count);
for(i=1;i<=50000;i++){
if(a[i]>=2){
printf("%d\n",i);
}
}
return 0;
}