239. Sliding Window Maximum

Sliding Window Maximum

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.

For example, Given nums = [1,3,-1,-3,5,3,6,7], and k = 3.

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

Therefore, return the max sliding window as [3,3,5,5,6,7].

Note: You may assume k is always valid, ie: 1 ≤ k ≤ input array's size for non-empty array.

Follow up: Could you solve it in linear time?

Solution

public class Solution {
     public int[] maxSlidingWindow(int[] nums, int k) {
        if (nums == null || nums.length == 0) return new int[0];

        int size = nums.length;
        int[] result = new int[size - k + 1];

        PriorityQueue<Integer> pq = new PriorityQueue<>((a,b)->b-a);
        for (int i = 0; i < k - 1; i++) {
            pq.add(nums[i]);
        }

        for (int i = 0; i < result.length; i++) {
            // add k+i
            pq.add(nums[k + i-1]);
            result[i] = pq.peek();
            pq.remove(nums[i]);
        }

        return result;
    }

}