From the above editorial For PROBLEM B can anyone please explain how the number of pairs will cnt*(cnt-1)/2
Not Able to understand
1420B-Rock and Lever
#include<iostream>
#include<vector>
#include<algorithm>
#include<ctime>
#include<random>
using namespace std;
mt19937 rnd(time(NULL));
int a[1000000+5];
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int t;
cin>>t;
while (t--)
{
int n;
cin>>n;
for (int i=0; i<n; i++)
{
cin>>a[i];
}
int64_t ans=0;
for (int j=29; j>=0; j--)
{
int64_t cnt=0;
for (int i=0; i<n; i++)
{
if (a[i]>=(1<<j)&&a[i]<(1<<(j+1)))
{
cnt++;
}
}
ans+=cnt*(cnt-1)/2;
}
cout<<ans<<'\n';
}
}