LeetCode-in-All

239. Sliding Window Maximum

Hard

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]

Example 3:

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

Output: [1,-1]

Example 4:

Input: nums = [9,11], k = 2

Output: [11]

Example 5:

Input: nums = [4,-2], k = 2

Output: [4]

Constraints:

Solution

import Collections

class Solution {
    func maxSlidingWindow(_ nums: [Int], _ k: Int) -> [Int]
    {
        // monontonic decreasing queue, storing indexes of nums        
        var queue = Deque<Int>() 
        var ans: [Int] = []
        for i in 0..<nums.count
        {
            // mantain window size no bigger then k
            while !queue.isEmpty && i - queue.first! >= k {
                queue.removeFirst()
            }
            
            // mantain queue monotonic decreasing
            while !queue.isEmpty && nums[queue.last!] < nums[i] {
                queue.removeLast()
            }
            queue.append(i)
            
            if i >= k-1 {
                ans.append(nums[queue.first!]) // max value in window
            }
        }
        return ans
    }
}