Hackerland Radio Transmitters

• Time : O(nlogn)
• Space : O(1)

int hackerlandRadioTransmitters(vector<int> x, int k) {
    sort(begin(x), end(x));
    int res = 0, cover = INT_MIN;
    for(auto& h : x) {
        if(cover >= h) continue;
        res++;
        auto ub = upper_bound(begin(x), end(x), h + k);
        --ub;
        cover = *ub + k;
    }
    return res;
}