D. Same Differences【1200 / 思维 哈希】

https://codeforces.com/contest/1520/problem/D

#include<bits/stdc++.h>
using namespace std;
const int N=1e5*2+10;
int a[N],n,t;
int main(void)
{
    
    
	cin>>t;
	while(t--)
	{
    
    
		cin>>n;
		for(int i=1;i<=n;i++) cin>>a[i];
 		long long int ans=0;
		unordered_map<int,int>mp;
		for(int i=1;i<=n;i++)
		{
    
    
			if(mp[a[i]-i]) ans+=mp[a[i]-i];
			mp[a[i]-i]++;
		}
		cout<<ans<<endl;
	}
	return 0;
}