【leetcode每日刷题】239. Sliding Window Maximum

本文链接： https://blog.csdn.net/m0_38103546/article/details/102634334

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: 

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?

需要用线性的时间解出问题，所以不能使用暴力的算法进行逐个比较。

import java.util.LinkedList;

class num239 {
    public int[] maxSlidingWindow(int[] nums, int k) {
        if(nums.length == 0) return new int[0];
        int[] res = new int[nums.length-k+1];
        LinkedList<Integer> queue = new LinkedList<Integer>();
        for(int i=0; i<nums.length; i++){
            while(!queue.isEmpty() && queue.peek() == i-k) queue.poll();
            while(!queue.isEmpty() && nums[queue.peekLast()] < nums[i]) queue.removeLast();
            queue.add(i);
            if(i >= k-1)    res[i-k+1] = nums[queue.peek()];
        }
        return res;
    }
    public static void main(String[] args) {
        int[] nums = {1,3,-1,-3,5,3,6,7};
        int k = 3;
        num239 solution = new num239();
        int[] res = solution.maxSlidingWindow(nums, k);
        for(int item:res){
            System.out.println(item);
        }
    }
}

使用一个双向队列保存当前窗口中的最大值的下标，当队首元素等于i-k时，表示窗口右移，不需要进行比较，则移除队首元素。否则比较将要进来的元素和队尾的值，如果队尾元素比将要进来的元素小的话则移除，然后将当前元素入队列，队首总是保存当前窗口中的最大值，每次获取队首的值放入结果数组中。

1,3,-1,-3,5,3,6,7 k=3

队列 i=0，queue =[ 0 ]; i = 1, queue 移除0，queue =[ 1 ] ; i=2, queue = [ 1,2] , res = [3];

i=3, queue = [ 1, 2, 3], res = [3,3]; i=4, queue = [2,3], queue = [4], res = [3,3,5];

i=5, queue = [4, 5], res = [3,3,5,5]; i=6, queue = [6], res= [3,3,5,5,6];

i=7, queue = [7], res= [3,3,5,5,6,7];

【leetcode每日刷题】239. Sliding Window Maximum

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