239. Sliding Window Maximum

class Solution {
public:
    vector<int> maxSlidingWindow(vector<int>& nums, int k) {
        vector<int> result;
        deque<int> d;
        for(int i = 0;i < nums.size();i++){
            if(!d.empty() && d.front() == i - k)
                d.pop_front();
            while(!d.empty() && nums[d.back()] < nums[i])
                d.pop_back();
            d.push_back(i);
            if(i - k >= -1)
                result.push_back(nums[d.front()]);
        }
        return result;
    }
};