Number of unique elements using c++ in o(n):
    /*author => shivamzaz
                            level=> ad-hoc
          complexity=> o(n)  */
    #include "bits/stdc++.h"
      using namespace std;
    int main(){
    int a[]={2,2,3,2},cnt=0;
    bool b[4]={0};
    for(int i=0;i<4;i++){
        b[a[i]]=1;
    }
    for(int i=0;i<4;i++){
        if(b[i]==1){
            cnt++;
        }
    }
    printf("%d\n",cnt);
    return 0;
    }