Same Differences Solution

#include <iostream>
#include <map>
using namespace std;

int main()
{
    // read number of cases
    int t; cin>>t;
    while (t--){
        // read n
        int n; cin>>n;
        // map a[i]-i to frequency
        map<int, long long> freq;
        long long ans = 0;
        for (int i = 0; i < n; ++i)
        {
            // read next int
            int x; cin>>x;
            // we can pair x-i with each previous appearance of x-i
            ans += freq[x-i];
            // increment frequency of x-i
            freq[x-i]++;
        }
        // print answer
        cout<<ans<<"\n";
    }
}