Sliding window maximum, k=3, input [1, 3, -1, -3, 5, 3, 6, 7]
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right=0 a[right]=1  deque(value@index)=[1@0]
right=1 a[right]=3  deque(value@index)=[3@1]
right=2 a[right]=-1  deque(value@index)=[3@1, -1@2]  -> window max 3
right=3 a[right]=-3  deque(value@index)=[3@1, -1@2, -3@3]  -> window max 3
right=4 a[right]=5  deque(value@index)=[5@4]  -> window max 5
right=5 a[right]=3  deque(value@index)=[5@4, 3@5]  -> window max 5
right=6 a[right]=6  deque(value@index)=[6@6]  -> window max 6
right=7 a[right]=7  deque(value@index)=[7@7]  -> window max 7

answer = [3, 3, 5, 5, 6, 7]