LeetCode - Sliding Window Maximum

Given an array 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.

For example,

Given nums = 

[1,3,-1,-3,5,3,6,7]

, and k =
3.

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

Therefore, return the max sliding window as 

[3,3,5,5,6,7]

.

Deque

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