Counting unique element in large array

Question

One of my colleagues was asked this question in an interview.

Given a huge array which stores unsigned int. Length of array is 100000000. Find the effective

Answer 1

Look at its variation that might help you to find no. of distinct elements.

#include 
using namespace std;

#define ll long long int
#define ump unordered_map

void file_i_o()
{
ios_base::sync_with_stdio(0); 
cin.tie(0); 
cout.tie(0);
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
}

int main() {
file_i_o();
ll t;
cin>>t;
while(t--)
{
    int n,q;
    cin>>n>>q;
    ump num;
    int x;
    int arr[n+1];
    int a,b;
    for(int i=1;i<=n;i++)
    {
        cin>>x;
        arr[i]=x;
        num[x]++;
    }
    for(int i=0;i>a>>b;
        num[arr[a]]--;
        if((num[arr[a]])==0)
        { num.erase(arr[a]); }
        arr[a]=b;
        num[b]++;
        cout<