239. Sliding Window Maximum

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Problem

You are given an array of integers 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 1:

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

Example 2:

Input: nums = [1], k = 1
Output: [1]

Code

class Solution {
    public int[] maxSlidingWindow(int[] nums, int k) {
        if(nums == null || nums.length == 0) return new int[0];
        
        Deque<Integer> deque = new LinkedList<>();
        
        int[] res = new int[nums.length - k + 1];
        
        for(int i = 0; i < k; i++){
            int num = nums[i];
            while(!deque.isEmpty() && nums[deque.peekLast()] <= num){
                deque.removeLast();
            }
            deque.offerLast(i);
        }
        
        int start = 0;
        
        for(int i = k; i < nums.length; i++){
            res[start] = nums[deque.peekFirst()];
            if(deque.peekFirst() == start){
                deque.removeFirst();
            }
           
            start++;
            
            int num = nums[i];
            while(!deque.isEmpty() && nums[deque.peekLast()] <= num){
                deque.removeLast();
            }
            
            deque.offerLast(i);
        }
        
        res[start] = nums[deque.peekFirst()];
        
        return res; 
    }
}