class Solution {
    public int[] maxSlidingWindow(int[] a, int k) {
        if (a == null || k <= 0) {
            return new int[0];
        }
        int n = a.length;
        int[] r = new int[n - k + 1];
        int ri = 0;
        // store index
        Deque<Integer> q = new ArrayDeque<>();
        for (int i = 0; i < a.length; i++) {
            // remove numbers out of range k
            while (!q.isEmpty() && q.peekFirst() < i - k + 1) {
                q.pollFirst();
            }
            // remove smaller numbers in k range as they are useless
            while (!q.isEmpty() && a[i] > a[q.peekLast()]) {
                q.pollLast();
            }
            // q contains index... r contains content
            q.offer(i);
            if (i >= k - 1) {
                r[ri++] = a[q.peekFirst()];
            }
        }
        return r;
    }
}