How to find maximum of each subarray of some fixed given length in a given array

Question

We are given an array of n elements and an integer k. Suppose that we want to slide a window of length k across the array, reporting the largest value contained in each window.

Answer 1

You can achieve O(n) complexity by using Double-ended queue.

Here is C# implementation

    public static void printKMax(int[] arr, int n, int k)
    {
        Deque qi = new Deque();
        int i;
        for (i=0;i< k; i++) // first window of the array
        {
            while ((qi.Count > 0) && (arr[i] >= arr[qi.PeekBack()]))
            {
                qi.PopBack();
            }
            qi.PushBack(i);
        }

        for(i=k ;i< n; ++i)
        {
            Console.WriteLine(arr[qi.PeekFront()]); // the front item is the largest element in previous window.
            while (qi.Count > 0 && qi.PeekFront() <= i - k) // this is where the comparison is happening!
            {
                qi.PopFront(); //now it's out of its window k 
            }
            while(qi.Count>0 && arr[i]>=arr[qi.PeekBack()]) // repeat
            {
                qi.PopBack();
            }
            qi.PushBack(i);
        }

        Console.WriteLine(arr[qi.PeekFront()]);
    }