239. 滑动窗口最大值

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
        // 移除k以外的元素
        while (!q.isEmpty() && q.peek() < i - k + 1) {
            q.poll();
        }
        // remove smaller numbers in k range as they are useless
        while (!q.isEmpty() && a[q.peekLast()] < a[i]) {
            q.pollLast();
        }
        // q contains index... r contains content
        q.offer(i);
        if (i >= k - 1) {
            r[ri++] = a[q.peek()];
        }
    }
    return r;
}

难点：