Thank you for the insight! Since many others are posting code for this problem, I'd like to suggest that c++ isn't too far from python this time :-)

int n, d;
cin >> n >> d;

vector<int> vals(n);
for (auto& val : vals)
    cin >> val;

auto exists = [&](int val)
{ return binary_search(begin(vals), end(vals), val); };

auto result = count_if(
    begin(vals), end(vals),
    [&](int val) {
        return exists(val + d) && exists(val + 2 * d);
    });

cout << result << endl;

`