Problem - 1520D - Codeforces

解析：

        统计 a [ i ] - i  

#include<bits/stdc++.h>
using namespace std;
#define int long long
const int N=2e5+5;
int t,n,a[N]; 
signed main(){scanf("%lld",&t);while(t--){scanf("%lld",&n);map<int,int>mp;int res=0;for(int i=1;i<=n;i++){scanf("%lld",&a[i]);res+=mp[a[i]-i];mp[a[i]-i]++;}printf("%lld\n",res);}return 0;
}