题解 | 谐距下标对

#include <bits/stdc++.h>
#define il inline

using namespace std;
using ll = long long;
using ull = unsigned long long;
using int128=__int128_t;

const ll N = 6e5 + 5, mod = 1e9+7, inf = 1e18;
const double esp=1e-3;
double PI=3.1415926;

il void solve(){
    int n;
    cin>>n;
    vector<int>a(n+1);
    map<int,ll>mp;
    for(int i=1;i<=n;i++){
        cin>>a[i];
        mp[a[i]-i]++;
    }
    ll ans=0;
    for(auto [x,y]:mp){
        ans+=(y-1)*y/2;
    }
    cout<<ans;

}

int main() {
    ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
    
    int t = 1;

    //cin >> t;

    while (t--) {
    
        solve();

    }

    return 0;
}