【題解】Codeforces 1321B. Journey Planning

#include <iostream>
#include <map>
using namespace std;
 
int n, a;
long long ans;
map <int, long long> mp;
 
int main() {
    cin >> n;
    for (int i = 1; i <= n; i++){
        cin >> a;
        mp[a-i] += a;
    }
    for (auto i:mp){
        ans = max(ans, (long long)i.second);
    }
    cout << ans << "\n";
}