Sliding window minimum - java implementation - code to study

SlidingWindowMinimum.java

/*second algorithm:

given an integer array, for example, [3, 5, 4, 6, 5, 9, 7], given an integer k = 3
minArray[0] = min{3, 5, 4}
minArray[1] = min{5, 4, 6}
minArray[2] = min{4, 6, 5}, 
...*/
/*  
 arr = [3, 5, 4, 6, 5, 9, 7]
 k = 3
 res = [3, 4, ]
 dq = [2,3]
 
 i = 3
*/
public List<Integer> minSlidingWindow(int[] arr, int k) {
  List<Integer> res = new ArrayList<>();
  if (k == 0 || arr == null || arr.length == 0 || k > arr.length) return res;
  Deque<Integer> dq = new LinkedList<>();
  
  for (int i = 0; i < arr.length; i++) {
    // compute min
    // popFront of deque while element index < (i - k + 1)
    while (!dq.isEmpty() && q.peekFirst() < (i-k+1)) dq.pollFirst();
    // maintain min;
    while (!dq.isEmpty() && arr[dq.peekLast()] > arr[i]) dq.pollLast();
    dq.offerLast(i);
    if (i >= k-1) 
      res.add(arr[dq.peekFirst()]); // ?
  }
  return res;
}