栈与队列 - 滑动窗口最大值

239. 滑动窗口最大值

方法一：

/**
 * @param {number[]} nums
 * @param {number} k
 * @return {number[]}
 */
var maxSlidingWindow = function (nums, k) {
    class MonoQueue {
        queue;
        constructor() {
            this.queue = [];
        }
        enqueue(value) {
            let back = this.queue[this.queue.length - 1];
            while(back !== undefined && back < value) {
                this.queue.pop();
                back = this.queue[this.queue.length - 1];
            }
            this.queue.push(value);
        }
        dequeue(value) {
            let front = this.front();
            if (front === value) {
                this.queue.shift();
            }
        }
        front() {
            return this.queue[0];
        }
    }
    let helperQueue = new MonoQueue();
    let i = 0, j = 0;
    let resArr = [];
    while (j < k) {
        helperQueue.enqueue(nums[j++]);
    }
    resArr.push(helperQueue.front());
    while (j < nums.length) {
        helperQueue.enqueue(nums[j]);
        helperQueue.dequeue(nums[i]);
        resArr.push(helperQueue.front());
        i++, j++;
    }
    return resArr;
}    

方法二：单调队列

var maxSlidingWindow = function (nums, k) {
    const n = nums.length;
    const q = [];
    for (let i = 0; i < k; i++) {
        while (q.length && nums[i] >= nums[q[q.length - 1]]) {
            q.pop();
        }
        q.push(i);
    }
    const ans = [nums[q[0]]];
    for (let i = k; i < n; i++) {
        while (q.length && nums[i] >= nums[q[q.length - 1]]) {
            q.pop();
        }
        q.push(i);
        while (q[0] <= i - k) {
            q.shift();
        }
        ans.push(nums[q[0]]);
    }
    return ans;
}    

方法三：分块 + 预处理

/**
 * @param {number[]} nums
 * @param {number} k
 * @return {number[]}
 */
var maxSlidingWindow = function (nums, k) {
    const n = nums.length;
    const prefixMax = new Array(n).fill(0);
    const suffixMax = new Array(n).fill(0);
    for (let i = 0; i < n; i++) {
        if (i % k === 0) {
            prefixMax[i] = nums[i];
        } else {
            prefixMax[i] = Math.max(prefixMax[i - 1], nums[i]);
        }
    }
    for (let i = n - 1; i >= 0; --i) {
        if (i === n || (i + 1) % k === 0) {
            suffixMax[i] = nums[i];
        } else {
            suffixMax[i] = Math.max(suffixMax[i + 1], nums[i]);
        }
    }
    const ans = [];
    for (let i = 0; i < n - k + 1; i++) {
        ans.push(Math.max(suffixMax[i], prefixMax[i + k - 1]));
    }
    return ans;
}