题解 | 谐距下标对

#include <bits/stdc++.h>
using namespace std;

int main(){
    int n;
    cin >> n;
    long long ans = 0;
    vector <int> arr(n);
    for(int i = 0; i < n;i++){
        cin >> arr[i];
        arr[i] = arr[i] - i;
    }
    sort(arr.begin(), arr.end());
    long long target = 1e9;
    long long count = 0;
    for(int i = 0; i < n;i++){
        if(arr[i] != target){
            ans += count*(count - 1)/2;
            target = arr[i];
            count = 0;
        }
        if(arr[i] == target){
            count++;
        }

    }
    ans += count * (count - 1) / 2;
    cout << ans << endl;
    return 0;

}