[LeetCode] Problem 239 - Sliding Window Maximum

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Given an array nums, there is a sliding window of size k which is moving from the very left of the array to the very right. You can only see the k numbers in the window. Each time the sliding window moves right by one position. Return the max sliding window.

Example

Input: nums = [1,3,-1,-3,5,3,6,7], and k = 3

Output: [3,3,5,5,6,7]

Explanation: 

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Window position                Max
---------------               -----
[1  3  -1] -3  5  3  6  7       3
 1 [3  -1  -3] 5  3  6  7       3
 1  3 [-1  -3  5] 3  6  7       5
 1  3  -1 [-3  5  3] 6  7       5
 1  3  -1  -3 [5  3  6] 7       6
 1  3  -1  -3  5 [3  6  7]      7

Note

You may assume k is always valid, 1 ≤ k ≤ input array’s size for non-empty array.

Follow up

Could you solve it in linear time?

Code

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public int[] maxSlidingWindow(int[] nums, int k) {
    if (nums == null || nums.length < 1 || k < 1)
        return new int[] {};

    int[] result = new int[nums.length - k + 1];
    Deque<Integer> deque = new LinkedList<>();

    for (int i = 0; i < nums.length; i++) {
        while (!deque.isEmpty() && i - k + 1 > deque.peekFirst())
            deque.pollFirst();
        while (!deque.isEmpty() && nums[i] > nums[deque.peekLast()])
            deque.pollLast();

        deque.offerLast(i);

        if (i >= k - 1)
            result[i - k + 1] = nums[deque.peekFirst()];
    }

    return result;
}