Sliding Window Maximum

Write `max_sliding_window(nums, k)` returning the maximum of every contiguous window of width `k`, left to right. max_sliding_window([1,3,-1,-3,5,3,6,7], 3) -> [3, 3, 5, 5, 6, 7] A heap gives O(n log k). O(n) needs a deque holding *indices* whose values decrease from front to back: before pushing, pop every smaller value off the back (they can never be the maximum again while the new one is in the window), and drop the front once it falls outside. Store indices, not values, or you cannot tell when the front expires.