LeetCode : 239. Sliding Window Maximum

Problem:

You are given an array of integers nums, there is a sliding window of size k which is moving from the very left of the array to the very right. You can only see the k numbers in the window. Each time the sliding window moves right by one position.

Return the max sliding window.

Example 1:

Input: nums = [1,3,-1,-3,5,3,6,7], k = 3
Output: [3,3,5,5,6,7]
Explanation: 
Window position                Max
---------------               -----
[1  3  -1] -3  5  3  6  7       3
 1 [3  -1  -3] 5  3  6  7       3
 1  3 [-1  -3  5] 3  6  7       5
 1  3  -1 [-3  5  3] 6  7       5
 1  3  -1  -3 [5  3  6] 7       6
 1  3  -1  -3  5 [3  6  7]      7
Example 2:

Input: nums = [1], k = 1
Output: [1]

Solution

class Solution {
public:
    vector<int> maxSlidingWindow(vector<int>& nums, int k) {
        vector<int> ans;
        deque<int> dq;
        for(int i=0; i<nums.size(); i++) {
            // remove element from dq if it is out of window
            if(!dq.empty() && dq.front() <= i-k) {
                dq.pop_front();
            }

            while(!dq.empty() && nums[i] > nums[dq.back()]) {
                dq.pop_back();
            }

            dq.push_back(i);

            if(i >= k-1) {
                ans.push_back(nums[dq.front()]);
            }
        }
        return ans;
    }
};